I think that at the end of the series, Deborah will retire and that's why Jean didn't like the ending.
Deborah got back everything she lost in the past, like her reputation and the late-night show, and now she's building a casino to cement her legacy. But in episode 6 she said she wouldn't do casino shows, so what is her intention? To fight for the late-night show again? To do another special? I think Deborah will realize she's already achieved what she wanted and will announce her retirement on the MSG show.
And I think that's why Jean didn't like the ending but thought it was perfect, because she and Deborah have something in common: Both are artists who took decades to be recognized and awarded.
But unlike Deborah, I imagine Jean doesn't want to retire. She and Hannah must be worrying about what they'll do now that Hacks is over, but Hannah is young and has time, Jean doesn't.