I will most certainly not hand something as vital to me as my notes to some unrealiable cloud provider that advertises "no commitments" and provides no guarantees for persistence!

A colleague fell for this kind of thing and lost everything when the service went bust.

That's fair. Our platform is built around markdown, and you already have the ability to export all of your cards at once in that format. Of course, you lose the structure that we provide between those notes (whether nesting or zettelkasten), but that is also something we are actively working on to make sure that your data stays portable and accessible forever (also working to include more export formats in general). It is difficult though, as no one else supports both multi-parent nesting and zettelkasten like Supernotes does, so your data does lose some functionality if taken off the platform.

One of our long-term goals is actually to try to make data ownership better than the status quo by integrating with other providers that would normally hold on to your data and instead get it back to being owned by you, all in one place.

Might I ask which service went bust? Want to make sure we don't follow in their footsteps! We're also very much trying to directly charge our users rather than relying on an additional business-facing offering to pay the bills, as we think the result is a more sustainable business (if everyone that is actively using the platform is paying for it, hypothetically we will not go bust). All of my notes (1000+) are on Supernotes, so I'm fairly invested in making sure they're always available.