That FM5 criticism was kinda nonsense, and so is the GTS criticism. I'd say that after the 1000 cars willy-waving, most people prefer quality over quantity.
I agree that the FM5 criticism was bollocks. It was a launch day title rebuilt for a new system with around the same amount of content that such games have had since at least PS2. There were no games with equivalent or greater amounts of content on the new generation, and no reason to expect that one should (or reasonably could) have been produced.
The GTS criticism purely for lack of content isn't particularly well founded, a good game is a good game regardless of content, but considering the time and market that it's releasing into (and the rather generous interpretation of how they count cars) it's at the very least a relevant point. There are several games with significantly more content releasing at almost exactly the same time, and as such it's a very valid point of comparison and discussion.
It comes down to whether people are willing to sacrifice car and track count (and customisation, and a few other things) for the graphics, Scapes and online experience that GTS is offering as their core gameplay. It's actually a fairly interesting discussion, one whose answer will likely differ a lot from person to person and that cannot be honestly had without acknowledging that GTS has significantly less driving content than any of it's contemporaries.
Just because GTS would have seemed quite good in 2013, doesn't mean that consumers and critics can't (and shouldn't) have a different opinion in 2017.
As far as the willy-waving, as always people tend to paint it in black and white. Writing 1200 cars on the box wasn't worth shoe-horning in PS2 assets and innumerable duplicates. On the other hand, 55 cars in Driveclub was certainly quite limiting. Anecdotally, there seems to be a range around the 300 car mark where any moderately well curated list will have enough cars to keep most people entertained until well beyond when they stop playing the game. Beyond that it's simply adding more variation and exciting things to discover for the budding car nerds amongst us.
Like most things in mass market design, the best tends to be a happy medium. And failures in that regard are not something that can be pointed at the artists or programmers, but only at the managers and designers that decreed that it should be that way. Polyphony made a choice with their car and track list, and it's now on the rest of the game to prove that it's good enough to make up for the lack of content that might otherwise become repetitive and dull.
We still haven't really seen what the core gameplay mode consists of, we've only seen licenses and missions, so the core gameplay could be anything from amazing to abysmal at this stage.