RVP and Rooney are arguably financed by the cheaper deals you listed. Welbeck might be on a high wage but that is still not crushing the bank considering his total cost. The money has been spent on RVP, Rooney, De Gea and Kagawa. As they are top of the league it is hard to argue that he has performed poorly with transfers.
THe whole league has gone to crap though, being the best of a ever weakening league isn't as good a thing as it sounds. There is a reason why in the past 2 years English teams are starting to be more routinely embarrassed, yeah, its still a cup competition and anyone can win, especially with anti football, playing "proper" football Chelsea looked much much worse this season.
A 5 year 85k a week contract for Welbeck(assuming 5 years) probably has a couple mil signing bonus and will likely take out at least 20mil from the club, despite him being basically rubbish.
Welbecks aren't what help you fund RVP's, Utd have bagloads of money and can do significantly better.
For example, making the choice to let Welbeck walk, and buying Michu on a 40k a week contract at 2mil would save you 10mil over a 5 year contract AND you have a MUCH better player.
Transfer fee's are the small part of spending, wages is where the real money goes every year, and every bad player you pay, when you can NOT pay them, is money you could spend on other players. Every decision like Welbeck adds up, 20mil over the next 5 years, its probably closer to 25mil with a signing bonus, and thats likely excluding appearance/goal/assist bonuses and a basic wage rise every year. 3 players like that and you're talking about Messi/Ronaldo level wage for someone.
Welbeck + Berbatov + Giggs + anderson was throwing away how much a year.
When Utd were their best, and when Arsenal were their best, there was no room in the first team for rubbish players, no chance of players who were really crap getting improved contracts, no massively overpaying in a desparate attempt to keep very average or poor players around. Two managers who were cut throat, who got rid of Beckham, Stam, Overmars, Petit.