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

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.