Most shareholders are pretty against cash hoarding and a reluctance to invest.
It's not £100m that'd never be seen again. It's £100m into an asset. He would add significant value via - hopefully - improved team performances. On pitch success is by far the biggest factor in the share price. Also, he'd retain significant value if we wished to sell him on in the future and he'd generate an awful lot of off pitch revenue.
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.
It has zero impact on the shareholder meeting, spending cash you easily have has zero impact on stock value. They have **** all say in how the money is spent too. They can ask questions, but they have absolutely no control on how the club is run.
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.

Manchester United