You may never be as big as Slack but there's probably a business there.
It's not impossible to compete but just much harder.
There are two companies backed by deep pocket investors who are happy to lose money, their products are straight sold at a loss because they desperately need market share gains to hopefully dominate and survive when they finally worry about profits.
Now the company I'm at is doing fine as we're small and such and we haven't lost customers to do the absurdly low cost pricing from the other two companies. But I know of others who have lost customers, nobody can match the pricing, and simply because they don't have the backing they will not survive.
It's very possible the two deep pocket companies won't survive either, but it seems like a fairly destructive pattern to have a market torn up ... when the driving force isn't even creating a stable company / what I would think of as typical market forces that help everyone.
A great example is Uber: they’re dominant but they have a ton of debt and neither their customers nor, especially drivers, have a reason to stick with them if the money is better somewhere else. They could conceivably make that up through superior logistics but if you thought you could do better, say in a particular geographic area where you can get drivers who know where anything is, it might be reasonable to compete if the absence of such a huge amount of debt gives you enough leeway to support an app development team and treat your drivers enough better that they’ll favor your clients.
gitlab and bitbucket ain't github, but as github started burning, they started picking up steam. github sold for $7.5 billion. If they can get 1% of that value, that's still $75 million. If they can do so on the cheap, maintaining control, that's not a bad return. I doubt something like gitlab requires more than 5 people to do well enough to achieve that, given patient, low-overhead investors. Done well, that's $75 million returns for a $10 million investment. If you toss the investor $50 mil, and each employee $5 mil, it's not bad.
On the other hand, as far as I know, patient low-overhead investors are a myth.