It's just that you repeatedly mention that your main aim is to have a service that will last for ever; but there's nothing different in your service (apart from your intentions) that will guarantee that. Basically, your asking people to trust you on that one.
But if it was Open Source, with good import/export features, then if it was threatened by closure people could just move their data to another service using the same software (techy ppl could even run their own server).
I'm sure there are already open source bookmarking services; why not just help one of those instead of reinventing the wheel?
Of course, the business model for making money from Open Source is completely different. But from your stated intentions, that's not your aim.