Yes, I think that is a valid point and many have cited Dein's departure as a factor in all this.
I think the consistent top 4 finishes are fairly impressive, especially given the money other clubs were throwing around. Many predicted that Wenger had us in a spiral of decline which would see us drop out but I think that level of consistency, while it shouldn't be the limit of our ambitions, is an achievement.
I just get the impression that some people see a huge difference in Wenger's ability between his first part of his time with us and the latter part. I don't see it like that. Maybe he lacks a little bit sharpness now, as others have said most people drop off in terms of ability as they get older, but I don't think there has been a big a change as some people imply. If anything it's because he hasn't changed while all around him has which has been the problem. Or a problem. The billionaires coming in were definitely a factor too.
I think we'd be better off now if we move on from Wenger. I do think it's a bit of a stretch if people think we'd have been in a better position had we changed manager 10 years ago. It's possible but not a given. And I certainly don't think that Wenger is so incompetent that any manager could come in and we'd improve.