Well, this is getting very interesting, which is what I hoped.

Roth was a danger to Michael because of his incredible resourcefulness. Just look how, despite being in a coma from a stroke and the bodyguard's effort to smother him, he still arranged to get out of Havana alive and recover--and then spring that brilliant perjury trap on Michael. He had to die.

Suppose Michael hadn't ordered that Roth be killed at the airport. He might not have had another shot at him:

Roth would have been taken into custody by the FBI agents. He could easily have made bail, but he knew he'd be a dead man on the streets. So, I reckon , he might have made a deal with the FBI to give them everything he knew about Michael and the Corleone Family (and he had decades of experience and knowledge to trade) in return for lifetime protection and lenient treatment. At his age, it'd be an attractive proposition. The FBI, humiliated by Pentangeli's recantation at the Senate hearing, would have jumped at another chance to pull Michael off of his "legitimate" pedestal, and send him to prison. Roth would have been a priceless resource for them. And, if Roth were properly protected, Michael probably wouldn't have been able to kill him before Roth ratted him out. So, it may have been that the airport scenario was the only way to silence Roth.

As for Michael not sacrificing his men: Recall that he told Tom in the boathouse: "If history has taught us anything, it's that you can kill anybody." That went for his own people, too. No one was indispensable, especially for ruthless, cold-hearted Michael.