I think neither are bad and it's actually preferable for both characters to be alive and well right now.
Kuma's suffered so badly his whole life... Wanting him to be dead after all this is like wanting Guts from Berserk to be dead after all he went through. And if he's going to die he should at least go out with a big bang, like killing Saturn or being a big part in his death, and making sure his daughter is free from the World Government's oppressive clutches for the rest of her life. But the general point is that he's survived through all the hardships, so him dying after all this would just be weird to me. Leaves a bit of a bad taste in the mouth.
As for Saul I think it's actually a sweet ending to Robin's backstory, it gives her more closure and helps her get over the trauma easier while still not ruining the tragedy and devastation of the Ohara incident. The important thing is that Saul was the ONLY one that was revived- Her parents are still dead, the citizens are all dead, and the island it self is long gone now. A lot of the stuff they worked hard for, is gone.
And that's really all that mattered. One giant guy who, iirc, wasn't even native to that island, surviving does not minimize the tragedy at ALL imo.