Thanos would need the power gem and a fanboy[Starlin included?] writing his character to be nearly as impressive as Monarch was in a straight up fight. Monarch trouncing the heros through countdown and Arena was that kind of showing, it was more impressive than fighting off Hulk, Captain America, Thing, Doom, etc, like he did weidling the Gauntlet. If you think Thanos is strong enough to do what Monarch did there, without a power-up, make a thread with him versus teams of GLs, Supermens, Captain Atoms, etc like Monarch did, and do it as effortlessly as Monarch did. See if others feel the same.
The fight between Prime and Monarch was a testament to how powerful Prime had gotten in that part of the story. Not how weak Monarch is. Prime in his own right has fairly much railroaded his compeition too[The entire JSA/JLA etc while not as powerful as here, slaughtered plattoons of GLs, nearly instantly, on a couple occasions while not as powerful as here...]
Monarch 'had everything planned out' and might have won his war against the Monitors, until Prime, a seemingly unstoppable wildcard, got in the way.
