It is a new thing, because the bench players are worse then before for sure, BECAUSE the players who are bench material only and were reserves in past seasons, are starting now. And former starting quality like Toure and Gallas are losing it.
That's more that Arsenal's starting XI isn't as good as it was and they have a lot of injuries..
I'm not sure the backup's are much worse.
If Arsenal could play:
Almunia
Sagna-Toure-Gallas-Clichy
Rosicky-Fabregas-Denilson-Nasri
Van Persie
Adebayor
Sagna-Toure-Gallas-Clichy
Rosicky-Fabregas-Denilson-Nasri
Van Persie
Adebayor
With Eduardo and Walcott as quality backup and all the guys like Eboue, Diaby, Silvestre, Song, Bentner on the bench, I don't see that first XI being much weaker than the Arsenal team of the last couple of years, but the bench is significantly worse.
If you go back five years, I would agree with your point that the standard of the starting XI has declined significantly.
