What happens when the PCs get rich? Mike and Roger raised the question on Improvised Radio Theatre With Dice not so long ago. (I get so many springboard topics of discussion from those fellows I should pay them a royalty.)
Of course, it depends if the character’s main motivation was always cash. A rock star or boxer or actor hitting the big time might suffer an existential crisis until they realize it was the art or the sport that they really loved, not the fame and fortune. They may even find they’ve lost by winning. Adventurers in fantasy fiction often undertake the quest for other reasons: glory, friendship, duty. Achilles and the others were no doubt looking forward to the payday when Troy fell – what ancient hero didn’t enjoy a bit of city-sacking? – but their reasons for being there in the first place were many and complex.
Conan becomes a king, and in The Way of the Tiger gamebooks the character goes from being a lowly sewer rat to the headaches of running a kingdom. I had a Tekumel character who was one of the lowest of the low. He struck it rich but that wasn’t nearly the end of the story, because in Tsolyani society there’s no real provision nor room for upstart commoners. The campaign only came to an end a long time later when he led his clan to the far corners of the world and conquered a kingdom.
Joining the 1%, even when that was genuinely what you wanted all along, could be the start of your problems. You don’t even have to be nouveau riche to attract the jealousy of the ruling class. Nicolas Fouquet made the mistake of outspending the King of France. He was arrested by d'Artagnan (no, really) and spent the rest of his life in jail.
Even if you keep your freedom, most societies have things that money can’t buy – especially the feudal societies of many fantasy campaigns. Sumptuary laws will prohibit you from aping your betters. Most interactions in the world will depend on custom, land, rank – all things you might obtain if you are artful and lucky, but never simply by throwing money at the problem.
It all goes to underline that you can’t say how vast riches will affect characters in the campaign until you know the society. The rich industrialist in Bleak House is treated very differently by the ruling classes than his equivalent number in Tono-Bungay, which is set just half a century later.
Retiring the character when their objective is reached is a perfectly respectable option. It’s not just over-stuffed coffers that deprive a character of their motivation. Any specific objective, finally fulfilled, may leave you wondering what to do with the character next. In both Eureka and There Will Be Blood the lead character achieves the high point of his life near the start and then must deal with the long wait for death. Those stories might be a tad contrived in order to serve the purposes of drama. While some people might be left rudderless by success, most of us on achieving one objective would set our sights on fresh goals. After Elon Musk has the perfect self-driving car, there’s always Mars.
Camus said we must imagine Sisyphus happy. Some find that a paradox, but consider it the other way round -- if Sisyphus didn't have the boulder, he wouldn't be content until he found something else to strive at. If you construct your characters around one movie-style objective then they will get there and have to hang up their spurs. Make them more complex, with multiple shifting goals, and they'll have more life.