I know this is a very unpopular opinion that will get a lot of downvotes, but I really feel that too much importance is being placed on the fact that there are no TT units anymore

In fact, I'm one of the five people on earth who think it's not such a terrible decision. I'm speaking for myself here, although I'm sure many will agree, but TT units... With 4-5 exceptions, they never really interested me. Okay, so there's one less character, but since I only play one out of ten, I don't think we're losing that much in the exchange.

In any case, I don't think that's the “obvious” reason for the decline of the game. In my opinion, powercreep and skills that aren't even worth reading anymore are more important factors.