They should be dropped again, but that won't happen until really late in the game. You can definitely reforge old equipments if you really wanted to after upgrading it, but the upgrades generally come so much later that, well...
There's some other questionable last words instead of Youmu's being bad, and most of the magic damage ones are only kinda so-so later in the game. Good for a free attack, but not actually much stronger than normal moves you can use, if even. Then you have stuff like Mokou reviving everyone and casting reraise on everyone who's alive, holy crap what?
How ALVing works is explained in depth here, including a chart of how much AEXP each enemy gives. At levels 9, 19, 24, the AEXP required to level up jumps by about 50% compared to the last level (so they're the breakpoints) 30 being about 33% jump and 40 being literally double, max is 50. After 24 it'll start taking hours of fighting to level up no matter what you're fighting (aka, even postgame enemies), and 19~23 is where you can expect to be when you beat the main game depending on how much you grind. ...my party went from 23~24 to 27~28 over the entirety of expansion, so, the scaling after 24 is just murderous. x.x Final skills are available at 34~36 (not counting ones of the most incompatible learning types) but even final area postgame grind takes hours to level up once in the late twenties, it's just dumb.