You bring up apps like Skype doing similar work in 2005, but Skype was barely out of its 2003 public alpha by then. Version 2.0 beta came out in 2005 and was the first version to support video, and only supported video calling between two people.
And you bring up things that are supposedly bad about Slack that are basically non-existent boogeymen. UI stutter, load time, and excessive memory use, I can’t think of any time any of these things have existed at all or noticeably impacted my experience on Slack on a basic low end laptop.
Those older apps like MSN Messenger and the original Skype didn’t actually do the things that Slack does now. I mean specifically multiple simultaneous screen shares plus annotations plus HD video feeds (with important features like blurred and replaced backgrounds, added by Skype in 2019) for all participants plus running an entire productivity app in the background at the same time.
Skype didn’t have screen sharing, at all, until 2009.
https://content.dsp.co.uk/history-of-skype
You call this situation “unjustifiable” but we would struggle to find any personal computing device sold at any price point that can’t handle the application smoothly. If I go back five years and buy a $200 mini PC or a $300 iPad or $500 laptop it’s going to run Slack just fine.
Specs are just arbitrary numbers on a box. It doesn’t matter that we got to the moon using a turd and a ham sandwich for a computer.
You can’t accept that the layperson doesn’t care that back in my day we walked uphill both ways for 15 miles on our dial-up connection. If it works, it works.