There's no point in buying an unrelated business that you're going to leave operating as usual - you might as well just invest that money on the stock market instead, or pay it out to your shareholders. So presumably if a company is buying a solution to X then it believes it can gain a competitive advantage by integrating X into the rest of its business.
Given that, I would've thought that the earlier you buy the thing, the easier it is to integrate it with the rest of your company and the quicker you start gaining the benefits of that. Waiting to buy the winner shouldn't be any cheaper than buying all the competitors in the field at the start, because it's no secret how everyone's doing. So the only reasons not to develop in-house from day 1 are if you're not as good at developing things as a small independent company, or there's some other advantage that you're missing, or the option value from the fact that small companies can go bankrupt on their own (but that's only really applicable if the business is capital-intensive).
The way I'd think about it is "would that business put any time into a partnership with Apple/Amazon if the chance came up". I guess you could argue that any business above a certain size would consider them worth talking to for some kind of cross-promotional deal even if that doesn't make a lot of sense on the face of it - Caterpillar? Hilton? But I think small focused companies outside of Apple/Amazon's core field wouldn't want to spend time on something like that. So B2B, non-tech, smallish companies - which by their very nature I can't name a lot of, but there's a lot of them out there.
In economic theory there is no benefit to diversification (which is what a company does when it absorbs another company) unless there are organisational improvements shared between the companies. If the acquired company is truly completely unrelated there would be no benefit at all as Apple isn't able to improve the running of an unrelated company. Being bought out by Apple might be a nice outcome for any company's shareholders (as they might get a premium on the their share prices), but technically there would be no improvement to the business in terms of expected revenue.
Given that, I would've thought that the earlier you buy the thing, the easier it is to integrate it with the rest of your company and the quicker you start gaining the benefits of that. Waiting to buy the winner shouldn't be any cheaper than buying all the competitors in the field at the start, because it's no secret how everyone's doing. So the only reasons not to develop in-house from day 1 are if you're not as good at developing things as a small independent company, or there's some other advantage that you're missing, or the option value from the fact that small companies can go bankrupt on their own (but that's only really applicable if the business is capital-intensive).