The victory at the end of Jedi was “tenuous at best”. Anyone claiming otherwise probably hasn’t watched the movies very closely.
“I shouldn’t have come, Vader is on that ship” Luke
“Don’t get jittery Luke, there are a lot of command ships” Han
We aren’t just talking about the regular Star Destroyers, but the ridiculously large Super Star Destroyers. Just a single one of those would be a match for the entire Rebel fleet… and there are “lots”.