The value that you put into the walled garden will eventually be locked behind some sort of paywall. Lest that sound too cynical, it wouldn't be a walled garden in the first place if that wasn't the basic plan.
Probably one of the mental blocks here is that your exposure to a walled garden is bounded by the value you put in it, and it doesn't seem like "a few guys chatting in a Slack channel" is a lot of value. And yeah, it probably isn't for your median open source project, even accounting for the ease of underestimating the value of that chat. The median open source project is having a good day if two people are in its chat channel simultaneously. However, if you're a much bigger project, and you've got hundreds of lurkers and active ongoing conversation for hours a day that constitutes the "real" design of the project, etc., it is dangerous to let that be in a walled garden for "free". But, still, only dangerous relative to the value of what is in there.
"Site gets shut down" is just one possible exposure. More concerning is the "site becomes not free after all".
My synthesis of all this is that, yeah, there probably isn't a lot of concrete danger in using Slack for your communication, but that the signal it sends may not be what some people would expect. But even that depends; an open source Slack plugin may as well use Slack, whereas a GNU project wouldn't be caught dead there, and there's a whole continuum in between.
"Why is the team developing the thing you find valuable getting compensated for that concerning?"
It's about the value they'd have that would allow them to negotiate a deal you probably wouldn't have taken if it had been offered to you up front, i.e., "lock-in".
But I blame the project for letting themselves get locked in, not the company for trying to make the money.
Open source projects are usually unfunded and to a first approximation have a budget of $0. This makes them a strange case.
Probably one of the mental blocks here is that your exposure to a walled garden is bounded by the value you put in it, and it doesn't seem like "a few guys chatting in a Slack channel" is a lot of value. And yeah, it probably isn't for your median open source project, even accounting for the ease of underestimating the value of that chat. The median open source project is having a good day if two people are in its chat channel simultaneously. However, if you're a much bigger project, and you've got hundreds of lurkers and active ongoing conversation for hours a day that constitutes the "real" design of the project, etc., it is dangerous to let that be in a walled garden for "free". But, still, only dangerous relative to the value of what is in there.
"Site gets shut down" is just one possible exposure. More concerning is the "site becomes not free after all".
My synthesis of all this is that, yeah, there probably isn't a lot of concrete danger in using Slack for your communication, but that the signal it sends may not be what some people would expect. But even that depends; an open source Slack plugin may as well use Slack, whereas a GNU project wouldn't be caught dead there, and there's a whole continuum in between.