You don't need to get an IPR, you just make an invalidity counterclaim. IPRs may or may not be desirable for a number of reasons.

TIL. We're talking about US law, right?

I've never heard the term "invalidity counterclaim" and I was in Google Patent Litigation where suits come in constantly. IPRs were pretty standard. Maybe the lawyers did use it and I just never heard it.

If something was going to trial, then "invalidity" was one of the possible defenses, as was "non-infringement."

Who are you making this claim TO? The court? The PTO?