TLDR: Got over his skis and mad with power and money. Decides to invade Parthia. Gets wrecked by horse archers. That ends up being typical for Romans, but this was the first-ish time that happened. Some of those captured legionaries may have ended up in China, though it is unlikely.
Not going probably never even crossed his mind. Social status in Roman society was very strongly influenced by military success. He was a previously successful general.
Historically (with all the accuracy you get when you summarize all of history) raising an army and not leading it was effectively telling everyone that you should be replaced by whoever did lead it.
I don't think the Western Romans ever really learned from it did they? The Huns ended up wreaking them pretty hard.
I know the Eastern Romans did learn at some point out of necessity by creating their own professional units and hiring mercs.