I think you'd have unintended consequences...
Hiking fees would simply reduce the people who actually get off their ass and take a walk.
The other unintended consequence is that political support for "forever wild" would erode if fewer and fewer people hiked, or even thought about hiking. But at least it satisfies the libertarian fantasy of everyone paying for everything individually.
Hiking isn't the same as hunting or fishing. Hunting and fishing require a lot more planning and equipment and knowledge than hiking, which is really only walking outside. Licenses don't do much to increase the cost or effort it takes to go hunting. A hiking license would just discourage the casual hiker, but that still has consequences.
In truth (sarcasm aside) hiking, backpacking, mountain biking etc are not different from hunting or fishing. If the state is gonna require a permit for one then there’s moral or legal reason not to do the same with the other. Just look at the downstate parks or the gunks if you think people won’t pay. They will. It’s not even that onerous- 6-8 bucks to park- less if you buy an empire pass.
The best idea I saw was putting voluntary parking meters at trailheads where people toss loose change in to help pay for trail construction & maintenance.
The WMNF has payment kiosks at its trailheads or you can buy a yearly parking permit. I think the trailhead parking fees are around $3 and the yearly permit is $20-25 or so? That still isn't enough to cover NH's SAR costs. The state also offers a Hike Safe option that allows people to contribute and not be charged if they need a rescue (assuming no negligence).
Some have suggested an equivalence between hiking and hunting. But I think there is a bit of a difference there as over hunting can decimate a population. The worst over hiking can do is contribute to some localized erosion, you generally don't run the risk of eliminating a species by over hiking. So it makes sense to me that hunting is a bit more regulated (and you need to pay for personnel to enforce those regulations).
I don't know the right answer here. There are valid arguments on both sides of the issue. You want to encourage people to get outdoors. Will a fee discourage people? The WMNF parking permit has not discouraged hikers, that is for sure. So who pays? It can either be a usage fee or included in the state budget. I am okay with it being a usage fee. If the fees are low, it will not discourage people from going. The parking fee seems to be a solid option... especially when it is very cheap annually.