Sure, some factions of the Rebellion likely stayed behind in case the second Death Star assault wasn’t successful, but clearly its leaders felt that they stood a good enough chance to blow up the Empire’s newest weapon (particularly since they had what they believed was top-secret information).
That doesn’t sound like this will be the case at all in Episode IX.
The Rebellion lost a lot of good soldiers on Hoth at the beginning of The Empire Strikes Back, but the fact of the matter is that a year after that conflict, the Rebellion had enough people among its ranks to wage an attack on the second Death Star, with Han Solo and Leia Organa leading the effort to destroy the space station’s shield generator, while Lando Calrissian, Wedge Antilles and Admiral Ackbar were among the many who took part in the space battle.
The Resistance may have grown back to being a credible threat, but going off of Oscar Isaac’s explanation, there’s no way they would have enough people to carry out an assault on the same scale that the Rebellion did three decades ago, and narratively speaking, that’s probably for the better.