To elaborate a bit further: Koscielny, Özil, Ramsey and Sagna belong to the top European tier. Mertesacker could arguably be put in the category this season. The rest of them can all be improved upon significantly. The problem is that 'enough money' today isn't the same thing as it was five or ten years ago. You're looking at £50-60 million for a top class striker now. You might get them for £15-20 million less if they have a release clause or if they are underperforming but that's where the market is. Even defenders are expensive. Guys like Shaw and Coleman who have proven nothing at top level won't go for less than £25 million. The great ones who already belong to a top club will go for a lot more than that. The difference in class isn't comparable to the difference in value, but it translates to a few points more on the pitch across an entire season. And those points end up making the difference.
Ultimately it's about how the players perform as a team of course, but there's no reason to think that better player won't perform better together. In short, practically everyone that we have could be replaced with a better footballer, but you need to pick and choose your fights because even the big money will only get you so far in today's transfer market.
