When have other clubs breached the top four, since 2000? Citeh, obviously. Everton for only one year. Tottenham a couple of times, when Liverpool have been run terribly, and Chelsea/Arsenal dropping the ball this season (if they stay in the top four, this season). Newcastle twice, but that ended well. Leeds once, but that ended well. To get in there, and stay, teams have to spend far more than they earn, it's just a fact... they can't get there, and stay, organically.
Everton never had the cash to invest in the squad in the first place
Tottenham ARE doing it without a lot of investment
Pardew is doing great work up at Newcastle, even with the numpty owner.
Wenger is doing it through sheer determination (even if he is a bit nuts for not investing in a few experienced players, as well as the youth he insists on)....and the fact that apart from Citeh, the EPL is getting weaker year on year
To be fair like most things its cyclical - ie for a time Chelsea was utterly dominant, just prior to that Arsenal were, and Fergie has been there for the most part trying to challange them both for best part of 3 decades nearly - but really owners needed to look at what was happening at Chelsea / Arsenal for last few seasons and target their places
Arsene has been the same for years (and the fabregas situation was always a matter of time rather than IF it was going to happen) , there has been yearly / every 18 month changes at Chelsea for the last 5 years or so - and they have been hanging on to top four slots because of little challange from elsewhere as much as because they deserve it (look at how the points for the title keeps on falling)
You can complain all you like about Utd, but SAF is one of hte few managers who has tried to invest in youth while still keeping a core of experienced winners in the side - a bit of a shaky start for the GK, but he is improving every game, a lot of young players who could have 5 - 10 years still at the club with a few promising U18's etc who should hopefully fill out the present squad in a couple of seasons time (although admittedly a few medium- high priced midfielders wouldnt go amiss right now)
If Pardew is left to his devices, with a few additions for his squad in the next two summer transfer windows , I think he could really make a good consistant impression on the top 4 before too long (depending on how long the Chelsea rebuild takes of course), Depending on what happens to Harry, and/if who comes in to replace him ...and who can be persuaded to stay at Spurs
Liverpool could easily be a consistant top four side again if they can convert a better % of chances they create (I hope not lol but they are proviing its about the small margins )
Right now I wouldnt be surprised to see a top four in a few years time consistantly without a London club at all - there are definitely spots up for grabs currently /in the next season or so without doubt (but plans at other clubs have to already be in place and be in partial execution to get it done in time)