These guys are solid across the board. They'd know to drop the bluffs and fancy plays and GTO strategy to just play exploitable ABC poker, because they'd know that's what wins at micro-stakes.
And when they start building some reads on their opponents, they'd adapt to exploit the weaknesses.
It'll be like playing vs a table of fish for them, even if it's a table of micro-stakes regs.
Or do you think they became pros by luck and not because of a deep understanding of poker theory?
They wouldn't play there. It would mess up their games. What sets these guys apart is that they accept higher levels of risk than most players. There is little risk when you are basically playing for bus change. Same with freerolls, little reward and no risk.
Yes, the best players can do very well at micros, but not necessarily better than the best winning regs at those stakes, since population tendencies vary widely between stakes, and exploiting population tendencies is how you beat the micros where you are playing against random players that you don't have a ton of stats on all the time.
I think they would maintain a great ROI in terms of micros, but we must also take into consideration that we have a lot of good players in micros Poker today.