Any club that sticks with a manager so long is going to spend much less than the benchmark.
The relatively new wrinkle to this argument is the suggestion that we've only spent such a small amount of money because we've had the same manager all this time. Okay ... so Stubbs came out recently and basically admitted Moyes wanted more money to spend and the board has said no. Yet by the logic of that argument we will spend money if we get a new manager. So why would they refuse to let Moyes spend money (with arguably the best track record of spending money in the league) but happily let a new unproven manager spend more money than Moyes? (Even if that were true do you think that's a logical way to run a club with such delicate finances?)
Spurs, RS etc. don't spend more than us because of new managers -- they spend more than us because they have more money than we do. So they spend it. One of the ways they spend that money is by firing managers a lot (which costs money to pay out a contract and costs money because of squad turnover from a new manager). This is a correlation/causation problem.
In addition people are making a faulty assumption that a new manager will have the "4th-7th best squad" in the league. They might for 13/14 but after that they will have the squad they build with no money. It's an odd assumption too because even most Moyes out types admit he's better than average with transfers. Better than average means something -- it means we're statistically unlikely to find a better manager at transfers. Not impossible ... but unlikely.
If we get a manager who is average at transfers we'll eventually end up with the 14th best squad in the league.* He'll then need to outperform that place tactically to get us back to 7th. Of course that just gets us back to where we are now.
What you want is a manager who outperforms his net+wages *and* outperforms the squad quality with tactics. You'd be hard pressed to find a manager who outperforms Moyes in the market; you'd also be hard pressed to find a manager who outperforms his squad by 7 places on a consistent basis. Finding someone who can do one of those things is tricky enough -- the vast majority of clubs fail to find someone who can do even one of those things -- but we need both. You essentially want us to hire the best manager in the world. So do I ... I just don't think that's especially realistic.
* If you want to get really technical I'd argue we wouldn't have the 14th best squad. We spend so much on wages because Moyes assembled a team which can support (to some degree) such high wages. A manager who is average (not bad ... just average) at transfers will miss out on a few of the players we pay highly. I don't believe that money immediately goes back into transfers because that money might not exist (if we finish lower in the league). We spend next to nothing (net) on transfers but spend a lot on wages which helps our squad.
Don't underestimate the importance of this distinction. Money on wages is lower risk than money on transfers. More often that not our big wages are on a player's second contract (Heits being an exception). So they are a somewhat proven commodity at that point and can be relied on to get us a decent league finish which in turn gets us more money.
To just assume we'd have the same spending level with a squad assembled by someone less adept in the transfer market doesn't make much sense.