Actually, they didn't just let Skype rot, they changed a bunch of things on it for the worse IMO. Skype used to be peer to peer (I believe the name is literally supposed to be a combination of "sky" and "peer to peer"), MS took that away to funnel it all through Azure. They redesigned the UI multiple times trying to follow the trend of whatever new app became popular (one was clearly trying the be a knockoff Snapchat). They forced all users to create Microsoft accounts to keep using Skype.

Not all their changes were bad, they did finally make a Linux client, which after many years became stable enough to use.