I thought about the XP and General one but decided that Monarchy alone wouldn't guarantee better trained troops. There may be some aspects of Feudalism that might go along with Monarchy that could grant that though.
Also have to consider it's weight compared to it's surrounding Gov't Civics. I was thinking I may have added TOO MUCH to Monarchy. I wanted it to be a bit better than Despotism, but not overly so to the point it would be better 99% of the time.
It is true that those time specific units will become obsolete. Unfortunately there isn't a system built in to grant Free Promotions for units made during a Civic, nor give a bonus to a specific class of units, such as Mounted. I would have to list every one out, and that would also mean they are all listed in the Civic. It gets messy with even just the Building list.
But that is something to consider. I DID have Riding School included, so it would fall in line with that. Hmmm...