in that case you have never seen a creature use Banishment on someone that dumped or even just passibled there Cha score.
twice in 1 campagain we had a cha 15 (far from dump) get banished round 1 for an entire fight... by the way it was our Barbarian (and main damage dealer). 1 of those two we were not on prime and as such it derailed the entire campaign as we had to go back home find him get him just to keep going with the adventure.
There are some major problems with that scenario.
First, How many monsters actually cast Banishment? Death Knight, Arcanaloth, Undying Councilor, Archmage, and maybe a handful I missed. Probably less than a dozen evil creatures total across every source book. If we aren't fighting celestials than that's really about it.
Edit: just saw your campaign premise. 13 Death Knights is a bad, bad day, but they only get a +5 to CON saves. End Edit.
Second, forcing concentration saves. Every. Single. Time. A caster takes damage they must make a DC10, or higher saving throw to maintain concentration. That may only be a 25%ish chance to lose the spell, but it happens multiple times every round.
Third, the range is only 60', so we don't need to worry about him targeting the Wizard, Rogue, or Ranger.
Fourth, Charisma is a common save proficiency. Clerics, Warlocks, Sorcerers, Bards, Monks (level dependant,) and Paladins are making this saving throw more often than not (+8 here vs a DC16 is 65% unbuffed, and those are conservative bonuses and targets for a lvl 10 PC.) The rest will have issues, but if a paladin is fighting alongside the vulnerable frontliners, the save chance increases to somewhere between 30 and 35%. Not great, but certainly good enough to risk for a couple rounds.
On top of all that, we have spells like Bless to buff saves, things like Bane to debuff them. Each throwing another 5-20% on the table.