Have we?
United have been to 4 cup finals in the last 8/9 years and several more quarter/semi-finals and have consistently performed reasonably well in the league on top of that.
I'll be honest; I'd take United's 'less' successful period over our own 2nd/3rd place finishes of late any day of the week. If we'd won the cup in 2011 or won something with the Higdon/Law/Ojamma team of a few seasons ago I reckon we'd have bigger crowds now.
Add in your last point to my points above regarding the young players and a decent argument for why they have better crowds is complete.
Success in the league is all well and good but if we can't follow that up by learning lessons of how to play in europe and kill off cup ties, I'm afraid without us winning a trophy soon, we'll see the crowds decrease more and more.
As an aside, I don't have a season ticket (can't afford it, nor am I able to make every game to justify it); I've been to 3 games (Fulham, Stjarnan and St Mirren) so far this season and spent over £50 just to get into the stadium (£10, £25, £19). Pricey.