> and dumping some proprietary lossless representation of their internal format doesn't seem unreasonably difficult.
I feel like you're not accounting for the time and effort it takes to maintain backwards compatibility. Once you give a customer a file that uses your internal format you have to always make sure your service will be able to read it back in. Going forward you aren't maintaining just 1 format, you're maintaining N different ones.
Good point. It would not be free, nor necessarily trivial/easy, to support this.
But does that mean it's unreasonable to require companies to do it? We already force plenty of engineering with regulation (GDPR, HIPAA, FERPA, PCI-DSS, SOX) because these are things society wants, even if the company would rather not.
Also, maybe the proprietary format doesn't need to work forever. Just add a disclaimer that it's only guaranteed for, say, 1 year? Or just "best effort"? All the file does is give the customer an incentive to return to Prezi later on. It would be in their own best interest to support this, if they weren't allowed to sidestep the whole issue by making it difficult to leave in the first place.
I feel like you're not accounting for the time and effort it takes to maintain backwards compatibility. Once you give a customer a file that uses your internal format you have to always make sure your service will be able to read it back in. Going forward you aren't maintaining just 1 format, you're maintaining N different ones.