Every few months, someone on your team reads a blog post about Slack's pricing or data policy and forwards the group a link to Mattermost or Rocket.Chat. The thread gets a few enthusiastic emojis. Someone spins up a test instance. Two weeks later, the conversation is back on Slack.

This isn't because the open source alternatives are bad. Mattermost is arguably a better-engineered product than Slack in several dimensions. Rocket.Chat does things Slack can't. Element/Matrix has a political story nothing else matches. So why does the migration fail?

Because Slack isn't really a chat app. It's an identity layer, a set of bot-mediated business processes, and a shared memory. The chat is the smallest part. When you migrate, the chat moves. Those other three don't.

The identity layer

Your company probably has a dozen services that use "Sign in with Slack" or that rely on Slack membership for access control. A Notion workspace that mirrors your Slack channels. A PagerDuty rotation that pings Slack. A Vercel integration that posts deploy messages. Zapier zaps that fire on @mention. GitHub's Slack bot.

When you move to Mattermost, every one of those breaks. Some have Mattermost plugins and most don't — your GitHub PR notifications, your Linear integration, your CI/CD status pings, your uptime monitor's alerts. You can rebuild them, but "we'll just rebuild all our integrations" is a quarter of engineering time, not a weekend.

What would make this better: a critical mass of B2B vendors shipping Mattermost integrations alongside their Slack ones. A few years ago, Mattermost integrations were a rare thing; now about half of B2B tools have one. We need the other half — especially Linear, Vercel, PagerDuty, Sentry, DataDog, and every modern CI/CD platform. Until that gap closes, Mattermost asks you to give up real operational capability in exchange for not paying Slack.

The bot-mediated processes

Most teams that have been on Slack for more than two years have at least one critical process that flows through Slack. Oncall handoff. Deploy approval. Customer escalation routing. Standup bot. These exist because somebody built them once, forgot about them, and now they're load-bearing.

When you migrate, these need to be rebuilt in the new tool. Rebuilding them requires finding whoever originally built them (who has probably left). Then rebuilding in a similar framework on the new platform. Then convincing everyone to trust the new process during the brittle handoff period. This is where most migrations die — not because the chat doesn't work, but because the duty rotation broke for a week and the team revolted.

What would make this better: better migration tooling. Mattermost's Slack importer moves the messages but doesn't move your bots. Same for Rocket.Chat. An "import your Slack workflows" feature — even one that just creates stubs with TODOs — would radically reduce the friction. Some of this is already happening through third-party tools, but it's not smooth yet.

The shared memory

This is the quiet one. Slack is where the team remembers things. "Oh, we talked about this in #design-feedback last September." When you migrate, you can import the history, but people don't search migrated history. They search the current tool's history, because the mental model is "if it's before [migration date], it's gone."

Three months after a migration, the team's shared memory has reset to the migration date. This loss is real and psychologically heavy. People who were comfortable in a rich shared space now feel disoriented in a new one. Tenure-related power dynamics reset: the person who knew "we tried that in 2023" no longer has receipts on hand.

What would make this better: this is harder, because it's not a tooling problem. The migration has to be paired with an explicit archival strategy ("old Slack is read-only at this URL for one year, then exported to PDF and stored"). Leadership has to repeat "yes, you can still look things up at the old URL" for about six months. This is dull work, not glamorous work, and it's usually what gets dropped.

What to do if you're still determined to move

If you've read all that and still want to migrate — and good on you; many teams should — here's what actually works.

1. Pick a moment of change. Migrations that coincide with an org change (a reorg, an acquisition, starting a new year) succeed more often than migrations that happen during business-as-usual. The disruption cost is already priced in.

2. Map your integrations first, then pick a target. Write down every Slack integration you rely on. Check each against Mattermost/Rocket.Chat/Element's plugin directory. Anything critical that's missing becomes either a build-it-yourself project or a veto against moving. If your Linear+Slack loop is sacred, and Linear doesn't have a Mattermost plugin yet, that's the whole decision right there.

3. Move in reverse order of criticality. Start with a channel that doesn't matter much. Let it live on the new tool for a month. Add one more. Migrate the general chatter channels. Keep the critical ops channels on Slack until the new tool has been stable for 90 days. Don't flip everything at once.

4. Budget 90 days before you expect the team to feel okay. Migration fatigue is real. First month is enthusiasm, second month is frustration ("this doesn't do the one thing I rely on"), third month is grudging acceptance. If you can't afford 90 days of reduced team happiness, wait.

5. Don't delete Slack. Not at first. Not for six months. Read-only is a perfectly valid state. The $7/user/month for a frozen workspace is cheap insurance against the migration rollback.

The honest recommendation

Most teams under 20 people should stay on Slack, invest the saved hours elsewhere, and wait two years. The ecosystem around Mattermost and Rocket.Chat is improving fast, and the day will come when the integration gap isn't a migration-killer.

Teams over 100 people with real compliance needs should move, but plan the migration like a product launch with a dedicated person running it for at least a quarter.

Teams of 20–100 in the middle are the hardest call. The savings are real (cheaper seats), the disruption is real, the integrations situation is genuinely getting better month by month. If that's you, pilot for a single team of 5–8 people for 90 days before committing everyone. If the pilot sticks, expand. If it doesn't, keep paying Slack and try again next year — the gap will be narrower then.

This isn't a post about open source being worse than proprietary. It's a post about the fact that the chat tool is the smallest part of the chat tool, and most migration plans forget that. If you want to ditch Slack, the question isn't "is Mattermost good enough?" — it's "are we prepared for six months of identity-layer and shared-memory friction in exchange for the eventual savings?" For some teams yes. For most, not yet.

Browse our team communication alternatives if you want to take a closer look at the options.