Let me be a little more constructive.
First let me clarify, in my mind the forum is quite separate to the main paid BPL site, I have no problem with paid access to the published content, which I think is a good model and excellent value. But the forum is entirely community driven, putting pay barriers up to this will break it (or change it beyond recognition). I don't buy the idea that restriction to members might improve the quality, like any online community there is a critical mass which can rapidly move en masse somewhere else. People will go where the conversation is, there's nothing to stop us all hanging out at at the whiteblaze UL forum tomorrow.
And even a nominal fee will drive off people, partly because of the hassle, but also because many people object to paying any amount for some things, on principle, because it transfers ownership from the community to the host. If you had to pay $5 a year to a company to edit wikipedia, it never would be where it is now. And what is BPL providing for that $5? A platform to communicate with others? The internet has many many places for this where you pay nothing. The argument that you need paid memberships to avoid spam is bunk, there are innumerable non-pay forums which surmount this issue without paywalls.
I suggest again that BPL take inspiration from the packrafting forms: create a separate hosted backpackinglight forum, sponsored and branded by BPL, but a free and open discussion space open to all. The traffic and interest generated into the main paid for published BPL content should provide some return for very minimal hosting costs. If that's not enough, put up a donation system and make the donation account transparent and clearly separate to any commerical BPL accounting. The advantage of having a non-profit open forum is that you can do this. I would happily donate $20 for example, whereas I wouldn't pay a mandatory $5 membership fee - can you understand the difference?. Similarly, you could pursue getting unpaid volunteer work for migration of this forum database over to a modern system - if you have to do that for your paywall forum attached to a commerical site with membership fees, you'd be expected to deal with that yourself, out of your pocket.
The alternative will be ending up with a much smaller member only discussion space for those who've paid for BPL membership for the published content. With respect to all of these people, I don't want to spend time in a smaller exlusive member only set. There are a ton of good contributions on here from non members, you can grandfather them in now, but that's just going to slow the fade, because what about the next 10 years of people who you might have had on here that you won't see now?