A lot of my thoughts re: Loop's envy towards Siffrin in post-canon boils down to that even if you gave them everything they theoretically could've want on a silver platter - give them back their body, a chance to reconnect with their Siffrin's party ect - they'd still not be happy and fulfilled because they've become such a different person from Siffrin.
And it feels awful to think that what was once so important to them just feels too painful and sensitive and complicated now. So the envy turns more into wishing they could still be happy with those things than simply wanting them back. They wish they could be Siffrin again, not Loop playing Siffrin's role.