I'm struggling to see the difference between using iron horn after iron horn and another caster just spamming say fear until the target fails a will save.
Saves scale for everyone, abilities does not.
Saves have a +20 soft cap.
Ability scores have a 12 soft cap.
Saves have easily available hard bonus feats to plug gaps, saves are more easily geared for.
This is why using abilities isn't a great target for spells, because they cause a very binary interaction and it's not fun being on the opposing end of that.
Pushing further towards RPS mechanics is terrible, things shouldn't have a too severe a disadvantage against others otherwise there becomes little point.
This is why, as you mentioned, bigby was given an additional reflex save and expanded immunities, despite it being weaker than iron horn. (And because you can't pray out of prone, which leads to exasperation of the initiation meta, see below)
Moving on, there are some niche counters, as you have reiterated needlessly.
This assumes that these counters are running, applied and have no down time and that the warlock himself can't do a thing about it, or that during the period warlocks won't do anything else. Even using the counter takes a round which puts you /way/ behind and you absolutely have to use it, as there's no pray net for people. If you're not initiating combat first, you're in trouble. These sort of meta-mechanics on initiation do not lend themselves to an RP environment.
The major point here, at least mine, is that the archetypes that are vulnerable to IH are also vulnerable to a warlock who doesn't use IH. IH is not needed. It makes them a very hard counter, and pushes into the RPS mechanics which are not fun.
FWIW infinite casting for /any/ save vs effective death spells (whether they target abilities or not) is bad. It's lazy and unfun. Feylock is marginally better simply because the counters to their particular brand of uninteresting spam are much more accessible and have a lower chance of working. This doesn't make them okay.
Thankfully this team is no longer being used.