One distinction worth holding onto, because it decides how bad a shutdown is: what is on the device versus what only exists in the cloud.
Most of these devices store a few days locally and immediately upload. So the historical archive is not on your wrist, it is on their servers, and your access to it is the app. When the app goes, the device usually keeps telling the time and counting steps and loses everything that made it interesting.
Which means the export is genuinely your only copy, and the deadline is real.
Going forward, a habit that costs nothing: export once a quarter and keep the files. Put a recurring reminder in. It takes minutes and it means a shutdown, an account problem or a switch is an inconvenience rather than a loss.
And on the interpretation, since that is what people actually miss when they move: the numbers are portable and the analysis is not. Sleep scores, readiness figures and training load are each company's own formula, and they do not transfer or compare between brands. Keep the raw measurements and accept that the scores end with the service, trying to reconstruct them is the part not worth your time.