The belief among many United fans is that the Glazers would be content with just finishing top 4. If this was true, why did they sanction a 24m deal for an injury prone 29 yo striker when we finished 2nd on goal difference the season before? The answer is because top 4 is not good enough. Sponsors want trophies, not just making it into the CL every year. Nike dropped Arsenal precisely because they were not winning anything. The same will happen to United if we don't keep winning trophies and the Glazers know this. They will spend the money needed to ensure United keep winning. They spent 24m on RVP to help reclaim the title, something they would not have done if top 4 was enough for them.
You may then point out that the Glazers have not forked out the money in recent seasons. Why? Because there was no need to. We had Ferguson, the only manager that could keep winning trophies with the players he had. There was no need to fork out money on superstars when the existing manager could win with the likes of Young and Valencia. But that's changed now. Ferguson is gone and any manager replacing him will need financial backing to win. And I'm sure the Glazers know this, as evidenced by the purchases of Mata and Fellaini (27m is big money), as well as giving Rooney a new contract.
They sanctioned the deal because they backed Ferguson in his pursuits and look at his spending previously. They give a transfer budget they think is fair and Ferguson can spend it how he wishes.
I reject the notion they will spend the money needed and in doing so compete with City and Chelsea to try their best to win the title. They will employ a manager who is an over achiever and give him the task of top four, in doing this such a manager will probably mount, consistently, a title challenge.
I wish they would spend £70m - £100m on transfers each and every season but I can't see them being able to justify that. They can justify a summer or two of spending big because of how small previous summers have been therefore that would serve to balance the books regarding funds available for transfers.
The RVP example is ridiculous when you look at the net spend. As any business they budget and Ferguson had a budget, in fact, he hardly spent in their first 5 or 6 seasons. In the last three he did, they do not tell him who he can and cannot spend the money on. They give him a budget and he decided 24m for RVP was a good deal, if you don't think so then so be it. Even the glazers would have seen it as a bargain, any business owner knows they must ensure adequate funding to ensure the business continues to evolves. There are other challengers for top four therefore even to achieve that requires spending and Ferguson really didn't spend much.
I also reject your opinion, as per the last paragraph, the Glazers tell Ferguson who he can and cannot spend money on. I am of the opinion he is giving a finite transfer fund and measurable objective to accomplish, he then can spend the funds provided as he wishes. The need to sanction a deal is merely to ensure he does not spend ridiculously (e.g. 100m for Bale) but otherwise they have no desire to tell him whether RVP is or is not needed. They also understand underspending in some seasons will likely mean overspending in others and such will balance out.
You also contradict yourself, you suggest there was no need to spend on superstars but with Ferguson's leaving that has changed but then you also stress the Glazers desire to spend on superstars like RVP. They let Ferguson do as he wished with the money, he wasn't going to spend when there was no need to knowing he could then ask for more in future seasons, where there was a need to.
I would like it to be so they will compete with City and Chelsea in future seasons financially because they think it will lead to more profit however I doubt this to be so. I think it makes more business sense to provide a budget capable of consistently getting top four and fairly task the manager with such however by employing an over-achiever, that manager will probably mount title challenges. That manager won't however get the sack for not winning the title.
Commercial sponsors don't sponsor winners specifically, they sponsor the biggest and most respected brands. They won't drop United after 4 or 5 poor seasons provided United remain a highly reputable and respected brand. I don't agree with the Arsenal comparison because their brand does not have the reputation as United but had it so then I doubt Nike would drop them. In mixed martial arts, fighters that don't win all the time but always come to fight in an entertaining way can earn a lot of money from sponsors because of their following. The sponsor would rather sponsor a fighter that isn't a winner but with a larger following than a fighter that is a winner but (probably because fans find him boring) a weaker following.
I firmly believe if United can maintain the strength of their brand then sponsors won't drop them. 10 or more seasons down the line the Glazers would have probably steadily reduced their stake in the club, it's going to take something like that time frame at least for United to no longer be a strong brand providing they can get top four consistently.