Rarely. Generally the teams that improve the most, win. City had weaknesses, identified them, won the league. United had weaknesses and improved by signing Matic, Lukaku etc. In the past Chelsea under Mourinho came close to the title, brought in Fabregas and Costa and won the league. Usually the teams who go out in to the market and decisively work on getting proper improvements are the ones who win titles. We obviously can't afford to spend like these teams, but we can afford to bring in players who can compete for spots. Dembele isn't suddenly going to have his best season at 31 after being plagued with injuries, we need to work on that.
I don't know what your point is, really. Do we have a good team? Yes. Do we have a team which cannot be improved? Absolutely not. The fact you say our depth is good makes me think you didn't really watch us all that regularly last season, we regularly had to rely on players who weren't good enough and I expect it to be the same this season. Compared to the rest of the top 4 our depth is poor.
Ok let's put it another way. Who do you think Spurs can buy that will make a significant difference to your league position?
Zaha, maybe? There is talk of Lewis Cook and Jack Grealish, who might be good for the future but are surely not going to make much immediate difference...
Let's face it. It could be perfectly rational not to buy anyone but fans like you will always hate it.