What do you mean "all the tracks"? All the tracks are currently in the game.
I get that it's a hypothetical, but it's also a false equivalency: dumping 4K support wouldn't have magically brought back all the tracks.
I may not be the biggest fan of the current small track list (or more so the weird decisions, like 3 ovals and a bunch of rally tracks), but I appreciate PD aiming for 4K. It will be a much more common thing at the end of this year, and 2019 even more so. Chances are there won't be another GT until at least 2019, so sticking with nothing but 1080P would've put the series in the same back-foot stance GT6 found itself releasing against modern consoles.
At some point, content has to be cut to pave the way for higher quality content. It happened with GT3, and that worked out pretty well.