>nor is there any good reason to just assume that favouring short-term gains will favour the current shareholders more
The reason is that it's a situation that's bound to happen. If I plan to be invested in a company for a specific length of time (for whatever reason) any decisions that benefit the company beyond that term do not benefit me. If those decisions actually harm the company in the short term then they work against my interests.
>Amazon could sell AWS for 5 billion dollars tomorrow
AWS isn't a product, it's capital. It'd be like a factory selling its machines. You only liquidate capital if you need cash right away, precisely because capital is worth more than its flat monetary value.
Why would the future value of the company not benefit you? Do your shares get stolen or cease to exist? I assumed you were to sell them.
The market need not agree with the company's management that the direction it's going will ultimately be beneficial.