Actually the main reasons to retrive Isha are so she can
1. Resurrect the five trillion dead eldar
2. Super charge their population growth which is one of the big reasons why the Eldar can't do nearly as much as they want to.
3. Resurrect the Slann
Even back when she was needed for Guliman and Lin, those were very distant 10s on the list of reasons why the Eldar at least wanted her back.
There's other things only she can do like fixing other eldar gods, resurrecting Astartes heroes like Dante and getting Guliman back to complete 100% for example.
The ritual is complicated, but it does have 1 guaranteed positive upside, according to the AO, it'll add a century maybe 2 if we're lucky until the Dragon feels it can crush the galaxy like an ant.