Thanks

I try to figure out how things work. I have some background with computer processing and code. (not a lot, but enough to be 'dangerous').
One thing people likely assume wrong is that replays are videos. It isn't YouTube. Replays are basicly just scripts. It is a set of instructions that tell phone clients how to play the actors (aka army units and defences) on your phone. It is less data intenseive to send a typed script/screenplay of a movie than it is to send the video of a completed movie. This is how many games run, sending directions to and from the server, moving abstract objects, and those objects turning into entertaining characters and enemies on the game screen.
This is likely why all our replays are gone after server maintenance. To make sure there are no glitches, old scripts are erased, because what is used to run those scripts could have been updated and changed, making 'words and instructions' in older scripts unusable, or liable to cause errors. Updating all those scripts is just more trouble than it is worth, and so just get purged. You don't usually need to save all of them, so they are wholesale erased on that assumption.