In that case it comes right back full circle to my point: to avoid having to bring down all the nails.
The fight never required Sentinel. It required a well-coordinated party to bring down all the nails to minimize Hellfire's damage. That's the clear and obvious design. A primal fight absolutely requiring a skill from a class to clear it would be absurd design.
Yet, as darkstarpoet put it before me, the community sets the status quo and this is not something the devs can always predict. Even in the community, it's not like this stuff happens in a planned or even controlled environment. The first teams to usually clear new content are decked out hardcore players. These are players that for the most part have every battle class leveled and are fully prepared for anything and everything.
So when they go into a fight like Ifrit, they don't have the coordination built yet to down all the nails. Anyone who had Sentinel, always had it on their bar. It's an amazing defensive buff for when shit hits the fan. So when they see Ifrit jump for Hellfire, they buff up. Of course they notice they survive with even only 1 nail down, so they have no further incentive to do this at that time. Keyword here is they. That team. The team that is made up of 8 players all with Sentinel. It works for them, they clear the content, they post their strat and everyone copies. Monkey see, monkey do. Very basic human behavior.
These guys accomplished what I wanted to accomplish. They did it like this. I will emulate them as an attempt to recreate their success.
This is the mentality that drives all the copying and the class-stacking and worst of all the idea that if you are not on specific class/jobs, then the fight is lost and not worth attempting.
Back when there were classes ad no jobs, everyone could potentially equip Sentinel. Having Sentinel made Hellfire nothing so long as even 1 nail went down. Bringing 1 nail down only gave people ample time to buff up and be prepared and required next to no effort. With a party full of Sentinels, it's the preferred method. If enough people in party don't have it, they could still coordinate a strat to bring down all the nails. You say this isn't incentive. Ask the person during 1.19 that didn't yet have Sentinel. Ask him if surviving Hellfire wasn't incentive enough to bring down all the nails. It was easier to ask for people with Sentinel. The hardcore crowd all had it after all, naturally.
Now we're here at 1.21, where with jobs having Sentinel equipped isn't dependent only on if you have GLA levelled or not. Jobs. They are better at what they do. There is massive incentive to use jobs in Ifrit - especially ones who, in a funny twist of fate, cannot equip Sentinel. MNK, BLM, BRD are very noticeable boons to an Ifrit party as DPS. None can equip Sentinel. So to use these jobs, you need to down I believe at least 3 of the nails. Downing all of them is greatly preferred for survival. Is there incentive to use jobs? Yes. Same reason there was incentive to stack LNCs and then THMs before them: smoother and quicker kills. Since there's incentive to use jobs, and the use of jobs requires bringing down more nails, then in turn there is incentive to bring down more nails.
I don't even have to argue it. Before 1.21 I doubt very many parties at all ever bothered downing all nails. If they did, they never even considered it their primary strat but rather something to try out. Now it's a regular thing.