Final tunnelling solution

So, make gens take 100, or even 120 seconds to solo (120 charges, to also make toolboxes less effective), and give survivors perma-endurance if the killer doesn't hook anyone else. Fair trade.
That would be the go-to solution to stop both gen-rushing and tunneling.

While the perk is definitely the furthest thing from the "final tunnelling solution" (no perk ever could be, for starters), and while it is in and of itself not giving survivors direct value (moving a hook stage from one survivor to another doesn't undo anything), there are some obvious use cases for this (and others that are a bit less obvious).

The most obvious one is having a survivor in struggle stage and knowing that the killer is likely looking to tunnel them. In this case you can take away one of their stages (and do this up to 3 times if everyone else has the perk). If you yourself did not have any stages yet, this means that instead of the unhooked survivor leaving the match on their next hook, the next hook will be struggle stage again (either with you on hook or them). Plus if the killer should decide to make you their new tunnel target, that can also be beneficial if you are better equipped to deal with that than the previous target, both in terms of experience/skill/etc. but also in terms of perks or items to combat tunnelling (DS and OTR for obvious ones, but also stuff like Camaraderie, Syringes, Styptics, …).

You can also bait a killer into making you their tunnel target by taking a stage from someone that isn't in struggle stage themselves yet, again with the idea that you can better live up to the challenge.

You can give a survivor back the potential of using Deliverance, Wicked or a luck-based self-unhook.

If you are the Rancor or STBFL Obsession or fully Condemned by Sadako or likely dying to an RBT or for some other reason either don't have to care about hook stages or aren't an attractive target for the killer, taking a stage can be done sort of "for free".

In the endgame it can be used to try and make sure that even if the unhooked survivor goes down again you can get another chance at saving them, then also potentially on a hook closer to the gate.

I do agree though that the downsides and especially the Exposed one are too much. The unhooker potentially being healthy if anything is the one direct upside the perk creates: now the tunnel target doesn't start the chase injured. But this is only really relevant if the killer does return to hook, because if they don't the unhooking survivor could simply heal the unhooked survivor for the same benefit. And if the killer does return to hook, the unhooker being healthy might not matter because they are Exposed.

I'm not sure there needs to be any drawback to this perk at all, hell, it could even come with a benefit to encourage anyone but premade groups to ever consider using this to begin with, as it puts the user at a significant personal disadvantage. I think one thing BHVR tends to forget when creating perks is that the perk slot investment itself is already a huge drawback: you only have 4 perk slots and you are forgoing using some of the strongest perks in the game if you opt for something more niche or situational or less impactful. Perks don't exist in a vacuum, they have to compete for perk slots with other perks.