I quite like the ending actually. A lot of people are so quick to bash things but as I've said before - reading arcs on weekly release vs in one chunk makes a world of difference and I expect it will be the same here
I think with Greenbull coming in the way they went out (btw both were unconscious before going into the Magma chamber) this actually serves three purposes:
1. Linlin and Kaido are ridiculously tough/durable, how else would they be disposed of permanently? Magma chamber seems fitting, especially since Wano is based on Tokyo and Osaka, and Mt. Fuji is just outside of Tokyo and is an active volcano - I had expected something volcano-ey to begin with and this is evidently lost on a lot of western readers
2. For Kaido specifically this is a poetic-justice moment. Kaido is killed in a Magma Chamber in similar fashion to the boiling oil he killed Oden with - this whole arc was about Odens' revenge/retribution, i only makes sense.
3. With Greenbull's involvement at the end of the arc I think the Magma actually acts as a transition phase - think of the arc like a powerpoint and the magma resembling Sakazuki and the Marines. Further to this, I think it demonstrates how dangerous Sakazuki actually is or will be and that these two Yonko were not as big a threat as the other top tiers coming later in the story (Sakazuki, Teach, Imu(?), Shanks, Loki(?) etc.). In 200 chapters or so this might be being praised in the same way Oda masterfully handled Doffy's pre and post TS involvements in the story which I recall most people were complaining about until the end of Dressrosa