I removed a major plot point from your question so that I can warn other readers here: I'm about to spoil the heck out of Season 2 of "Yellowstone." If you haven't already seen it but plan to, you should stop reading.
With that out of the way, I can now say that you're right -- the bomb that Kayce planted on the Becks' plane at the beginning of the second-to-last episode was made moot by the fact that they went ahead and shot all the Becks in the finale instead.
It's hard not to think that, amid all the tense things going on in the last few episodes of "Yellowstone," the writers just sort of forgot about the plane. (There was also a kidnapping, a couple of fathers dying, and a lot of relationship drama to get through in just a couple of hours.)
Of course, that's not to say the plane plot was wasted. One conflict that ran throughout this season has been the one within Kayce Dutton's soul -- will he grow into a wise and kind man, or a vengeful and hard one? His decision to plant a bomb in the plane was a big swing toward the bad side.
But there's still hope for him. The season ended with Kayce's tearful reunion with his son after rescuing him from danger, and Luke Grimes, who plays Kayce, said in a teaser video for Season 3 that the upcoming season will be a chance for his character and family to "heal" after all this violence.
So stay tuned. Season 3 is already in production and is likely to air next summer, in keeping with the past two.
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