That people use it and may like it, is not really 'lock in'.
Ultimately, it's an easy-to-reproduce experience, and there will be/are competitors offering 'the same thing' - and so there will be price competition.
If you want to 'switch to an alternative' it would be a disruption, not a 'massive pain'. Your 'old chats' are not that valuable.
If Slack was integrated into other systems - or integrated with partners, customers etc. - that is 'lock in'.
FYI - It may very well be the 'next cool things' comes along, and you find small teams 'just using that cool thing' already.
Finally I would say that Slack is not even that optimal - there is a lot of 'noise' in those channels, and the value add I believe is not that high.
I consult for startups sometimes, they put me on Slack and it's total noise. I ask to just use email, and it works.
I think 'best practice' is not in a tool, it's in behaviour.
Email, used properly, with the help of a simple chat app for teams, can work just fine.