As discovered by Pavel Djundik, data miner and mastermind behind SteamDB, Valve is working on a native feature to allow users to change their boot videos.
Valve is planning to add these so-called "Steam Deck Startup Movies" to the Steam points shop, where users can currently also purchase themes for their keyboards. You get these points by buying games on Steam and participating in events.
However, the method to replace the startup video is currently a little cumbersome as you have to replace a specific file in a hidden directory. Adding this functionality as a native feature to the Steam Deck is one way to assure people won't mess too much with critical components of their operating system.
But this makes one thing very clear: Valve keeps listening to the community!