They are right for now, but will start loosing clients soon. Zulip & Discord provide better plans without major feature set downgrade.

We tested Zulip not too long ago, in short - UI is not so good, but in general still useful: https://nixsanctuary.com/zulip-review-through-slack-glasses/

Zulip and Discord will charge more once their growth plateau, as every one else.

What's the cost of pissing free-tier people off? None of this has really impacted paying customers, right?

Some percentage of free tier people are also paying on other accounts, and some will be lost.

But it’s probably a small amount because SaaS has figured out to basically milk slacktivism. People will complain but pay.

I'd think that's not the main problem -- it's instead fewer new users? Impacting future paying customers

Opportunity cost to an extent: instead of convincing them into becoming paying customers, and having them word-of-mouth promote you, they slip away to a competing service, which they may also indeed decide to pay for.