If new startups agreed that they wouldn't leave their users out to dry if they got acquired would you be more likely to trust them and use their service?
Getting a future promise from a startup CEO is as trustworthy as a politician's promise during election season.
The only sane approach is open data. Leave everything available for export so some other feature company can pick up where you left off.
You could venture into an insane realm where startups enter into a Startup Insurance™ league. When a company gets acquihired, instead of shutting down the product, a separate foundation runs the product (or figures out a way to open all the data for export).
Keeping the product alive could be one of the terms of the acquisition, but it seems unlikely - it could lower the offer, and it's unlikely startup founders would throw away money on a matter of principle unless the community promise was somehow legally binding.
In a sense the promise is a "non-credible threat" in game-theoretic terms. It would attract users if they carried it out, but because acquisition happens after the user-attraction stage they don't have an incentive to carry it out. This means they won't rationally carry it out, which means they don't attract rational users. This situation can be avoided if they "tie their own hands".
If startups want to be acquired, they'd never make such a policy anyway, because that makes them less attractive to the acquirer (who, after all, may only acquire them to prevent competition).
