And how did those insane emperors meet their ends? Caligula, Nero, and Commodus, three of the biggest candidates for "eccentric emperor ordering quixotic expedition", all seem to have died violent deaths after upsetting someone. What army is going to go "well, this nutcase who happens to be named Caesar is telling us to travel over the open ocean to some made up place, guess we have to do it even though we can easily arrange to have him killed, or just kill him ourselves. Too bad us Romans have never been known to stab people..." The Emperor isn't some divine being whose authority is so supreme they have no choice but to obey their every whim. They're basically more akin to the president, and here's still a Senate, and the Senate still has some power and a voice. No senator is going to approve this. And an emperor who threatens the senate is an emperor who finds themselves to be the victim of Praetorian knives.