I still don't understand what I'm supposed to use instead, as a consumer. Are Google users just not allowed to IM each other anymore? We have to switch to Google's Slack clone?
It's a big pity. Even though it's a walled garden, Google technologies have the advantage that at least most people have a Google account. To replace it, I have Whatsapp, Signal and Telegram running on my phone. I need all of those (and messages.android.com) running on my PC.
I guess Google couldn't figure out how to monetize it.
Your link says, and it is also my understanding, that this only affects GSuite users, not the majority of Hangouts users. Is there any indication that most people will be affected ?
Yes. "Hangouts Classic" goes away for GSuite users Oct 2019, and they get migrated to "Hangouts Chat".
"Hangouts Classic" is going to be shut down at some unspecified date after that. All users will eventually be migrated to "Hangouts Chat" - GSuite users are just getting pushed first.
https://arstechnica.com/gadgets/2019/01/the-great-google-han...