> Is it justifiable to use the service of a possibly great startup, knowing that there's a real possibility that it might be sold soon and just be a gear in some other company's product?
Asking this question plays into the hands of the tech giants. Undermining the startup ecosystem ultimately benefits them even more than acquihiring.
I think it's a valid concern. My knowledge of startup culture is very limited, so from what i read here, the usual desired outcome is an "exit" aka getting rich by selling the business. I think that is totally valid for social sites and other freebies, but not for infrastructure services. there is a lot of potential and great ideas in things like hosting, monitoring, crm/helpdesk software etc, and if you're not entering the market with an intention to last, you're actually playing in the hands of tech giants. They're asking a lot of money for crappy and downright ridiculous services (ever worked with e.g. HP Service Center or anything SAP), but give you planning reliability in exchange.
With the given startup mindset, you're yielding that market to tech giants.
Asking this question plays into the hands of the tech giants. Undermining the startup ecosystem ultimately benefits them even more than acquihiring.