The real significance of the MAGA movement, as I understand it, is that Trump can deploy it in safe (often gerrymandered) districts in primary elections against Republicans who step out of line, with varying degrees of success.
That is true, but it's not the whole picture. Trump held sway over many Republicans because he could create a very real threat to their personal lives, even their physical safety. If he turned his ire on someone, MAGA responded and harassed not just politicians but their family. There was no advantage to crossing him.
No one else in the Republican party has that kind of personal charisma (I don't see it, but it's obviously there to many people) to sway the MAGA to madness. Remove Trump and his threats and Republicans may start responding to the threat of their general angry constituents instead.
If JD Vance steps into the presidency, will he continue to rule with the same autocratic cruelty? It wouldn't surprise me if he tries to do that, but I'm doubtful he can get away with it. He reeks of elite snobbery and disdain for his roots, the very opposite of Trump's faux man-of-the-people vulgarity.
My hope is that the different power factions under the Trump umbrella just fracture into in-fighting, and at the very least simply stop wreaking deliberate damage to the country.