Geordi isn't a cadet, he's a commander with over two decades of experience. He was on the command track in the early years of his career, before switching to engineering in 2365.
He was only a Lt JG on the command track by the time he transferred I think. And while he's been a Commander for 20 years, he's been based in the ships' lower decks for that time.
To suddenly become responsible, full-time
, for a Galaxy-class ship is too much of a stretch.
I like the idea of Geordi becoming a Captain to tie in with Voyager, it would give him a lot of development, but I wasn't convinced with how it was done here.
And the Challenger was an SCE ship, a vessel crewed almost entirely by engineers and dedicated to engineering-related missions, so it made as much sense for Geordi to be its captain as it did for Scotty to be its captain, or for Sonya Gomez to be the da Vinci's captain.
After watching televised Trek
so many years, with its fairly rigid Starfleet personnel structures, a Galaxy class engineering-only ship...captained by Scotty...it's just fan fiction.