Heinlein was a fan of the forever war. But I do.not think it would have changed his take on starship troopers.Much before that, I'm really curious what would happen if you interviewed Heinlein in say, the aftermath of the Vietnam War, about how his thoughts had changed in the aftermath of a brutal grinding attrition war. The reason I say that is I think the society he depicts in Starship Troopers would get smashed against a tree, and otherwise crushed, by an attrition war, because its politics would collapse. He would obviously still be in favor of a volunteer force, but, I think Vietnam would give the middle finger to the idea that a powerful conventional force would be enough, and that occupation wars wouldn't be necessary. Which matters because I just don't buy the society he writes there would function in a long, grinding attrition war against an enemy that refused to give it a conventional fight.
The Forever War does deconstruct the book I realize, but we didn't see Heinlein react to the Vietnam War himself, but I suspect it would've made him question his ideological views on quite a few things.
As for tyranny burning itself out, I'd say that's broadly true, even though current times make me hesitate with that. History seems to suggest nothing, good or bad, is permanent.