They ran Skype into the ground by compromising on the quality of the end user-experience - just like they did with MSN Messenger - and the current desktop Skype client is an Electron app (at least this way Linux still gets supported...).

Skype was in prime position to be the best way to have free voice and video-calls between iOS and Android phones and the wider Internet (seeming as FaceTime is Apple-exclusive and Hangouts seems unpopular) - I don’t know what happened but WhatsApp ate Skype’s lunch and got bought by FB and the rest is history.

Skype on my iPhone X is just painful. I use it to make cheap PSTN calls when I’m abroad and simple things like switching to Speakerphone mode takes 2-3 seconds and involves opening the iOS audio output list (which has an animated opening for some reason) instead of every other app which has a single speaker button. This is but one example, of course.

They could have bought Slack, but they competed with MS Teams instead - but doing-so awfully because they still don’t have multi-tenant support in the client. Thing is - I can’t say that competing with Microsoft has made Slack any better (case-in-point: last month’s always-on WYSIWYG editor debacle).