Wenger was managing in Japan, Ferguson in Scotland, Moyes in the lower leagues. The odds might be against us but there are good managers out there who would love a chance to come to an english premier league club for the prestige and the wages and the exposure and the quality of the players.
If Moyes decides not to renew his contract, there's no reason for it to be disastrous as long as the board hire the right replacement. And I believe that there are both better and worse managers than Moyes who would come here, though obviously mroe of the latter.
It's the same as when anyone leaves the club from Rooney, Lescott and Rodwell to Moyes, Fellaini and Kenwright. Whether it's a good or bad move for the club depends almost entirely on who we can find to replace them. Until we know that, you can't say.
But the best clubs spin the wheel and don't act scared of change. See the way Ferguson and Wenger know when to move their best players on.