He gets to torment Gretchen and Elliot one last time and extort millions from them out of personal revenge.
Actually he didnt extort a dime from them, all he did was hand over his remaining nest egg ( around 9 millions) and tell them to "donate it" to Flynn in his 18th birthday so that the whole family can be millionaires and not worry about money for the rest of their days (unless Flynn donate it all to a streamer, which, wow, wouldnt that be a kick to Walt's dead nuts).
Anything related to the process of getting the money to Flynn were to come from the nest egg, not a cent from them two. Oh yeah and if they didnt do what he told them, there were going to get killed by top hitmen (which realistically Gretchen and Elliot should be able to hire the finest private security to investigate and confirm that shit was bogus but hey, why deconstruct one of the rare finales that doesnt fuck everything up?)
Walt got the happiest ending by far. He gets to leave his family millions of dollars in clean money. Visit his wife one last time who seems relieved to see him and even covers him against the DEA and police. He gets to torment Gretchen and Elliot one last time and extort millions from them out of personal revenge. He gets one final rampage against people whom he hates which also secures his blue meth formula from ever being recreated. He even gets to kill Lydia and taunt her over the phone for more personal revenge. He gets to make amends with Jesse by freeing him. He knows that he will become an even further public legend by dying fighting a Nazi gang by himself.
And finally he's going to be dead of cancer within the month if not week so him going out in a blaze of glory is no sweat. Walt's ending is him going out on his own terms. Cancer doesn't even get to kill him. Walt probably felt the best in his life as he lay dying in a lab thinking about how Jesse was happy to see him and his family will be wealthy for generations.
Yeah, people need to understand Walt...didnt give a shit about nearly anyone outside of family (Jesse was sort of like family after a point). Sure, he will be remembered as a monster by everyone but he legit doesnt care as he much rather be remembered at all than being forgotten after enough time passes. He also made it clear he doesnt care that he will go to Hell if there is one, simply he wont go there "laying down".
Felina was literally the best case scenario for him.
There was a reason he died with a soft smile while a cheerful music plays.
Because in the end, he did "won" in his battle against the world. Even the script says so.
I just think at that point they had to do superhero-level writing for Walt in order to justify his talent, Jesse figured it out why couldnt one of Gus' 50 scientists?
Walt has the recipe though, and it's apperantly easy enough to cook in a trailer. So you can make high quality meth but the cost to price ratio wouldn't cover it and the amount of heat it will generate will be immense.
It's an acceptable justification why no one else could do it, and why Walt needs to be kept alive until someone else knows his recipe.
The show establishes, not sure how accurate it is to real life but still, but it does establishes that percentages matter when it comes to purity. Gale told his 96% may seem fine as is but that the gap between 96 and 99 is "significant" (ironically this talk would eventually get him killed).
And even people that "know" Walt's recipe cant seem to reach the 99.1% somehow, maybe implying that Walt had a style or technique that only he could pull it off. It gives Walt value to the narrative and the characters as killing Walt will also kill the 99.1 purity meth.
Mike was a lost soul before the show even started.
I always got the vibe Mike knows he is damned already if there is a Hell so he might as well go the extra step on providing for his son's daughter before he leaves this world. This is why Mike comes off as pretty emotionally dead most scenes, only really softening up around characters like Nacho and Jesse (since he sees them as young men that made several wrong choices but arent too far gone that they cant wisely step away from this life).
He not only disappointed a noble son that idolized him (in a way, just like Walt with Flynn) but "breaking" him still got him killed. After the "half measure" with the wife beater and this, Mike probably concluded the world is a wretched wicked place (and he is no better) so he might as well go all the way for the closest thing that remains of Matty, his daughter.