BNP gotta have a downside

More often than not, a coordinated 4-man is winning against a Killer. If they bring 4 BNPs, they just hasten that conclusion. They didn't need 4 BNPs to win, so if anything, that is doing the Killer a favor by not dragging out their suffering. Even then, each BNP is only 5% of the total gen progress needed, or 20% total with all 4. Even current Pain Res is essentially equal to that. That is only 1 perk slot for Killer, but a forced item and addon for the Survivor to do the same. That seems about even to me.

Also you ask if Killers have addons that give a free hook. Well if you look at how long a chase lasts without and with an addon, you can get a percentage. An exposed addon can give you essentially half the chase time, since they only have 1 health state. So essentially an exposed addon gives you half a hook each time you get value. That means they have exactly what you are asking for (more often than not). To be fair, some Killer addons do nothing to their power, like Nemesis for his whip. But others have an addon to break pallets in 1.5s, (Blight Iri serum) give a Floods of Rage activation, (Ghostie Iri Camera), Exposed (numerous Iri), and plenty of other chase shorteners.

The main problem is in how the Pig who brought box skillcheck addons isn't in the same league as a predropping BNP spamming squad. Then the next match when she brings Iri tape Ruleset 2 for the Survivors to spawn with the headtrap and not see boxes until they are active, the solo squad falls apart immediately. Part of helping that would be having a visual indicator at the very least for the rarity of items. That way the Killer seeing 4 Brown Toolboxes doesn't overreact and brings the sweaty loadout expecting Commodiouses. They have the system for Event items, so honestly just color coding the item with the rarity might be good enough. Even if the item itself isn't visually distinct, the loading screen and escape menu (where you can check offerings) could show the rarity and class of item. (Eg. Green Key, Iri Map, Brown Medkit, Yellow Toolbox.) It shouldn't show addons though. (Unless maybe as a buff for Hoarder.)

There are 5 total gens needing to be completed. Completing a single gen (which takes 4 BNPs) is completing 20% of the total match gen requirement as you need 5 gens to pop to start working on gates. Divide that by 4 for the total gen requirement (again of the match, not the gen itself) per BNP, and you get 5% total gen time for the match. They also are typically burnt ASAP. Which doesn't typically help to have gens done early, as later when you need that final gen is when it is most valuable. Losing 5% of the total match gen time can matter, (to a single BNP) but not much more than sufficient pressure and short chases can provide.

It is important to keep the entire match in mind. It would be like saying "I just get hooked once and died." That would be skipping 2 hook states, or 16.66% of the game. (2/12 total hook states or 1/6) That can happen basekit right now if someone never got hooked until the hatch race of a 3K that could turn into a 4k. So a Killer can skip nearly 3 BNP equivalents by playstyle, not even an addon, and an Iri addon at that.

I don't think BNP's are healthy for the game. They should be revised or removed, but that´s only my opinion

While playing which killer kit?

When generators are completed faster than normal, but pallets are still being consumed at the normal rate, the entire match becomes a lot safer to play through since most killer kits can't burn through enough resources to make a palpable (or even reasonable) difference before the game is at 1 generator left with 80%+ resources still remaining.

Short of slugging everybody as soon as possible or "camping," certain killer kits can't do much to counteract the effects of the time cut that multi-stack bnp can provide.

Gonna disagree. I see far more iri myers than BNP. Also, in all the time I've played DBD I've never seen a full squad of BNPs. You get one every once in a great while. Even when I used to play with SWFs we still only had BNPs on rare occasions.