Your comment makes it sound like locking a bootloader down is an ultimately pointless exercise. But Microsoft got everything they wanted by doing so, so it was a complete success for them despite it getting hacked 12 years later.
The goals for locking down a game console are:
1. Preventing game piracy. Microsoft stopped Xbox One piracy for the entire effective lifespan of the console and forced people to buy their games at retail prices. Big win.
2. Preventing the Xbox One from getting turned into the cheapest PC on the market, like the 2001/2 Xbox hacks did and which must have pissed off Microsoft’s hardware vendor partners. Ok, now you can change out the operating system. But over the past 12 years PCs have gotten exponentially faster, so you’ve just spent a ton of time liberating shitty old hardware.