I don't think things are anywhere near as bad at Arsenal. Players like Valencia, Nani, Young etc were always competent but never considered in the elite bracket of their position in the Premiership, so there was always the risk they'd perform to an average level under an average manager. They were there to supplement the real stars. Those that were in that elite bracket, the likes of Ferdinand, Vidic, Evra, Giggs, Van Persie etc are all clearly well past their best and age catching up on them was always a matter of time. Some of the young players that looked like they had potential under Ferguson (Cleverly, Jones, Smalling and Wellbeck) have turned out not to. The only genuinely top class player that Moyes inherited who is still at a good age is Rooney. Maybe I'm a little harsh on Van Persie as he's been injured a lot this season.
At Arsenal, even if none of their latest generation of youth team players (Sanogo, Gnabry, Zelalem etc) turn out to be good enough, the likes of Wheelchair, Oxlade-Chamberlain and Szczesny stagnate and don't improve over the next couple of years, and the supplementary players (Podolski, Flamini, Giroud etc) all regress under a new manager, the - I'd still say that in Mertesacker, Koscielny, Ramsey, Cazorla, Ozil and Walcott they've got a strong enough spine of players who are all still at or before their peak that have proven themselves to be amongst the best players in their position in the league.
Sadly, they now also appear to have money to spend. I think we really missed the chance in the previous two seasons to overtake them but it's going to be hard now as, barring a major **** up, the foundations are there for more long term stability in the top 4.