This is a valid point. I've observed or been involved in a number of acquisitions at various distances over many years. There are any number of reasons an acquisition might not go ahead and, of course, as the potential acquirer you obviously learn some things that are useful, but I've never known a situation where there has been a deliberate plan to simply mine for knowledge or whatever.
The reality is some acquisitions are opportunistic, some are strategic, and even the opportunistic ones often have a strategic element. For a strategic acquisition, if it doesn't go ahead (comes down to ROI isn't perceived as being as good as potential alternatives), the almost inevitable outcome will often be (i) a different acquisition is eventually made, or (ii) the acquiring company decides to make an investment in that area themselves.
One of the ways to avoid getting "screwed over" as an acquiree is to ensure you've done the work beforehand to maximise the chances of compatibility with the acquirer: things like compliance, data protection, having a poor grasp of your numbers and financials, and other mundane matters (or combinations of them) can easily trip up the process.
When an acquisition does fall through for almost any reason it's pretty natural for the potential acquirees to feel rather bruised by the process: they've wasted their time, they've been screwed over, etc. Often that won't be the case although, I've no doubt, there are instances where it will be.
(Btw, in case it's not obvious, I know nothing about the activities of Stripe or its founders, good or bad.)