[Suzerain of Sheol]

Well, you raise the crux of issue, actually. And I'll explain in a bit more detail, since I'm probably the only person here who's ever read that book.

Basically, the War of Wrath at the end of the Silm wounds the continent of Beleriand so badly that it fractures apart and sinks into the sea. Belerian was basically the north-west of Middle-Earth, if can picture an entire continent springing from where the Gray Havens are, more or less.

The revealed power of the Valar and the Maiar, against Morgoth's Balrogs and Dragons was enough to shatter mountain ranges and break the land apart, so I can see why they'd refrain from going to war again.

But here's the thing: Sauron isn't Morgoth. Sauron is only a Maiar, barely more powerful than a Balrog, if at all. The Ring doesn't actually enhance his personal power to any meaningful degree; that isn't what it was designed to do. It's actually made him weaker in a few ways, as by binding his essence to it, each time he's subsequently destroyed, a bit of his personal power is permanently lost, which is why he can no longer shape-shift, which was his signature ability in the Silm.

What I'm basically saying is that the Istari, if they hadn't been weighed down with a plot device to prevent them from doing so, should have been able to march into Bara-Dur, beat Sauron senseless, and drag him back to Valinor in chains to go wallow in Mandos for a few hundred millennia. Honestly, Curunir and Olorin alone (Saruman and Gandalf for those of you playing at home) should have been equal to the task. And conflicts between Maiar are using only on the level of city-smashing collateral damage at best, and really, who cares if Mordor gets laid to waste. It already is a waste.