The players you've mentioned - and Sissoko - are squad players in the Spurs team, or at least Son was been until his recent good form has earned him a slot in the starting XI, which he'll probably keep unless and until his form drops away. Wanyama has also been starting games, but it remains to be seen if it'll stay that way now that Dier has returned from injury.
As I said in the summer, Spurs didn't especially need to strengthen their first XI, because they would likely only get better anyway due their young average age. So we concentrated on improving the squad depth (and bringing 2 or 3 of the more promising youth players into the senior squad), which I think we've done, and can now field a pretty decent 2nd XI.
You say we lack some top quality players, but I regard at least 4 of our entire back 5 as being top quality (Vertonghen makes up for being a bit less so because he gels so well with Alderweireld). I also regard the Dembele of last season (and hopefully he'll repeat that form this time around), plus Eriksen, Alli and Kane as all being top quality. That's 8 of our first XI, with the rest of our first XI being pretty good.
Of course this begs the question, how is "top quality" defined? If you are meaning players who would find a place in any starting XI in the world, then that would be a ridiculously high barrier. Or if you are meaning players who would find a place in some top team or other, then that depends on which clubs are defined as having a "top team" and what players they already have in each respective position, but I'm sure that some Spurs players are better than their equivalent in Man. City or PSG or Juve (for example) and vice-versa.
