DrJawaPhD
Hero
Correct answer here is that it totally depends on the world the DM is running. In a Low Magic world where PCs are some of the only spellcasters, it's a huge stretch. In a High Magic world where NPC Adventurers of higher level than the PCs are commonplace, not remotely a stretch at all. For worlds somewhere in between, TBDI think it’s a stretch to think that every random encounter is going to include a spell caster who has Dispel Magic.
Also technically it wouldn't be "every random encounter", it would just be every encounter that the Players realize occurred. If random monsters without Dispel Magic would just wander off and the only monsters the Players actually deal with are the ones with Dispel Magic, then yeah by default every encounter includes a spell caster with Dispel Magic.