Skype is the epitome of technical debt. Millions of lines of code for a service that isn't technically difficult to provide anymore. When I was at Microsoft, I was told working on Skype was about as popular as being sent to a gulag. The value of the brand is so strong, I am surprised they never launched a "2.0" version built from scratch and without all the vestigial tails.
I'm surprised that you are surprised!
Rewriting a million-lines-of-code project from scratch without the stupid bits is easy. Getting the equivalent of the working bits, instead...
Joel expressed this concept quite well already 25 years ago: https://www.joelonsoftware.com/2000/04/06/things-you-should-...