Yen Sid is meant to be this very wise and formerly powerful keyblade wielder but we know very little of his history or abilities. He basically just sits behind his desk in every game and does NOTHING. KH3 showed that he still has powerful magical abilities, yet he continually sits there and chooses not to help.
Firstly, we don’t know anything about his history as a keyblade master. We are told he is a friend of Eraqus but that’s it. Where was Yen Sid trained? If he was a powerful keyblade wielder, who did he battle and what are his actual accomplishments? What is his relationship with Xehanort? Was Yen Sid involved in the Dark Road stories? We don’t even really know where he comes from - possibly Disney Castle?
Next, we are told he is a retired keyblade wielder. Since when is someone a retired keyblade wielder? Did he retire because of his age? Can he still summon a keyblade? His abilities in KH3 showed he is still a valuable fighter, even if he’s not as strong as he once was. So why retire? Xehanort was very old and still a powerful keyblade wielder - physical ability doesn’t seem as important. Sora was the size of a 10 year old in the first game, Kairi is a weak teenage girl (sorry just being honest), Mickey is tiny, and Xehanort is like 70...so Yen Sid shouldn’t use age as an excuse to retire.
Finally, Yen Sid does nothing to help in the modern games. He sends people on dangerous missions and then just sits at his desk. He didn’t even help train Lea and Kairi and just made Merlin do it. I assumed Yen Sid was old and couldn’t physically contribute, but KH3 showed he has some powerful magic. Same thing with Merlin - they need to be travelling around worlds and using their magic to help out more.
Yen Sid should have been given more importance in the game’s history.