Or just tech in any minion that spawns shitters for your opponent and screw up their entire game plan.
Pretty much this. If res priest starts to become dominant, the meta can easily shift around that. Zul'Drak Ritualist is a fair counter but some niche counters such as Polymorph, Hex, Plague of Murlocs etc really throw a wrench in res priest's viability. As far as the ritualist, it will only start really seeing play if what you say comes to pass and res priest actually becomes dominant enough to warrant it. I personally doubt that'll be the case but if it happens, there are already counters available.