It doesn't "explain" it at all (and indeed many of reject the idea that MS apps - in general - are worse. They've had a bad reputation for decades, and that isn't based on nothing).
The new Office suite are unbelievably buggy and slow not because of backward compatibility, but because of numerous useless "features" Microsoft added which made the software essentially bloatware. By the time Excel opens up on my Mac, I can open several Google Sheets and start working.
Office seems fine to me - not great software, but I haven't noticed any particular decline (not a heavy user, but have been using it since Word v2 in the 1990s)
And in any case what does this have to do with Typescript and the (over?) use of types?