Are there any army wrecking battlefield enchantmets, which stay after the caster retreats from the battlefield?
Any of the damaging/debilitating battlefield enchantments would, near as I can recall, though most of them wouldn't do much if the caster was the
only allied thing on the field before retreating. Wide spread poison would tick away for a bit after a retreat, but it isn't likely to do much to anything except bog-standard human armies, and even then from what I recall the actually battlefield wide one(s?) take some time to ramp up (so an immediate retreat would be fairly suboptimal). Otherwise SL covered most of it pretty well.
Though to the reason for crippling instead of killing... it can be a nasty thing to do to enemies units with an upkeep, especially if you can manage it en masse. A bunch of blind sacreds or whathaveyou are pretty close to gold-sinking weights around your opponent's neck, and they have to sink resources/attention into either fixing them or getting rid of them. Diseased et al casters and whatnot can cause your opponent to do possibly silly things in an attempt to eke out a bit more use before they die, and feeblemind or whathaveyou is always amusing to drop on that 200+ gold/dozens of gems commander if you
can't actually kill them. Softening up an encroaching army or SC with afflictions can also lead to your opponent overestimating what they can accomplish when the
real battle comes. Lot of good reasons to spread the affliction love around, providing you can manage it and don't have anything better to do with your resources.
And in the end, a lot of times it's just that much
easier to cripple something than it is to kill them outright, and then you can either deal with them later with much less effort, or laugh as your foe is stuck with the now-useless things. The latter's particularly fun, honestly. There's few things quite as
amusing as a giant death-stack showing up at your castle's door, half blind, all diseased, and starving, promptly routed by a force a fraction of their size. Not even killed, but set to running and still sucking on the teat of your foe's coffers and scattered to the winds to boot. There's
lamentations when that happens, shadow.
Lamentations.