People don't often think of their local development environment as a "platform" that their stuff needs to work in, but it really is. In that sense, unless you're hosting off of your laptop (please don't!), every app is multi-platform.

Every startup I've worked at (and I've been at this for 15+ years) has moved hosting providers, but I still wouldn't put it high on the list of reasons to avoid vendor lock-in. If you make sure someone(s) know how the app actually runs, and you try to pick stuff you can run locally, the vendor lock-in stuff won't be your biggest challenge in the move.