Teams is unique as a combination of chat, phone and SharePoint client. Thus it was born an abomination and the laws of nature dictate that it must only get worse.
Not just chat clients, but pretty much any stable piece of SW from a major corporation that reaches peak perfection, is forced to get worse and user hostile over time, because those VPs, execs and managers in charge of it want a promotion or need to justify their bonuses on a yearly basis, so they force needless changes to have something to present to the board every year till they jump ship or retire early with their masive stock piles.
And then the next people who take over, have something to tear down and rebuild, so they can be the ones making their careers on "fixing" that product and the cycle of shittiness repeats.
They are not products anymore that need to serve the best interest of the customers, but fiefdoms and vehicles designed to propel and fuel someone's ego and career advancements at all costs.
It's an effect of perverse incentives in corporate promotions. You'll never make a career at these companies by doing QA or saying "well, the product we have was perfect already, so I did absolutely nothing to it, added no new features, just fixed some bugs and kicked back".
Microsoft is a little different as they never really rebuild. Outlook is basically the same as it was in 1995, including non-functional search. :)
The only exception was driven by my employer, who opted to embed Cisco Call Manager crap into Microsoft OCS (the circa 2010 chat client). I would rather communicate via requisition notes in peoplesoft than use that shit. Whatever replaced it (iirc Lync for Enterprise Business Extreme Edition) was better.
I was working for a large company using Office 365 back then. Our teams were using either Slack or Skype. The company pushed everyone towards Teams because it was included in 365 only to eventually backpedal and move everyone to Slack because Teams was an unfinished software. What's surprising to me is that they have had a few years to improve Teams yet its UX is still far behind Slack.
I do believe the lockdowns might have rushed the transition as the UX felt unfinished for years. I can't think of any other reason that might have pushed them into replacing Skype so quickly.
Alternative venues include the default installation of windows to (almost) every laptop in existence.
AI/speech recognition - nuance communications (the stuff behind apple's siri)
side note: does linkedIn count?
These two products dominate every non-technical industry. I think MSFT makes mediocre software. But as long as those two gravy trains exist, they'll never lose.