Warmaster Horus
Explorer
In my game it would be pretty hard to cast Dispel Magic with regard to Banishment as you'd need to effect the target of the spell - not the spot it was cast upon as it was NOT cast on that spot, rather the creature that occupied that spot and is now on another plane. There are already lots of ways to avoid Banishment - counterspell the casting, make the CHA save, disrupt the concentration of the caster. Deal with it.