Yeah, it's an investment into an asset. But terms of a football club, an investment also means another asset needs to be sidelined and will start losing value. This isn't like a normal business were your different properties continue to generate revenue even after the latest acquisition. Unused $30 million players generate nothing but costs.
Of course they don't have any say on footballing matters, but they have plenty to say about your finances. Shareholders of a publicly traded company expect a tightly run ship. You can't seriously be thinking United are now going to start hoarding 50 unused players just because they can easily afford that. If you make a world record transfer, there's bound to be a clearout. It's just how it is.