They aren't asking to use an existing ongoing spell with War Caster, they're asking if they can change the target of a spell after its initial casting if said casting was via War Caster.
The answer to which is a solid no.
AoEs are fine if you can choose who or what is affected in the AoE.
Is there any evidence that reasonably maintained real world weapons broke or wore away to uselessness quickly enough that implementing time/usage based weapon breaking rules even makes sense?
Because initial searches are coming up with weapon 'lifespans' of a couple of years or more, which...
The vast majority of the actual first party books (core and otherwise) functionally don't exist for most players (and DMs) given how few people actually read them, so it's really hard to be sympathetic here.
From the spell itself:
And so:
3rd level slot: Animate 1 Skeleton (2 with Undead Thralls) or Maintain control over 4
4th level slot: Animate 3 Skeletons (4 with Undead Thralls) or Maintain control over 6
5th level slot: Animate 5 Skeletons (6 with Undead Thralls) or Maintain control over 8...
Not sure who started the 18 skeleton thing. Multiples of 4 (8, 12, 16) are better.
Anyway, having 18 Skeletons RAW isn't as difficult as you described.
You really only need to be a 7th level Necromancer if you're willing to work more than a day on it, 8th if you can wait a short rest, or 9th...
If my skeleton squad makes me so unbalanced that the rest of the party is irrelevant, then I'm clearly slumming around.
Normally, I'd take that opportunity to manage the less interesting but necessary stuff so that they can handle what most interests them with a sort of safety net.