I've never understood that either, especially immediately post take over when we were regularly referred to as 'the richest club in the world'... it doesn't mean we have the richest fans.
It makes me laugh that when we were in the lower leagues we were praised for our support and now it's used as a stick to beat us with. What's our crime, having the audacity to take on the established top sides? Doesn't that just make the league more interesting? The Abramovich Chelsea takeover helped break up United and Asenal's period of dominance and City's takeover added an extra challenger (Liverpool don't count, they haven't won a title since the premier league began and only the presence of biased ex players in the media has managed to keep them relevant for this long.
Opposition fans know we had great support during the bad times, it's a lazy criticism with nothing to back it up. That said we still need to come up with a better come backseeing as it's a regular occurrence at home games these days.