I'm trying to see this from all the angles. I'm certain that Spark's costs have gone up but like someone mentioned, a good deal of that certainly must be due to increased traffic. I think we can all agree though that this place has improved technologically a great deal over the years. Reliable infrastructure is not cheap.
That being said, people are feeling the crunch. It's not easy for me to come up with $40 every year for my membership, which is entirely for support of a good place; I don't sell anything here. Well, part of it is so that I can 'say' I'm a "knifemaker."

But that ain't worth forty bucks, much less seventy five. I won't renew if the price goes up and I suspect that my attendance here might diminish as well. "Registered User" does not ring many bells in my ego.

Perhaps many of those who have been around for a while won't ante up. The last price increase was hard for me to swallow, and almost doubling it will be the deal killer for me. Any increase actually, would be a deal killer for me. Having a cool tag by my avatar is not rationally worth much money.
But the thing is, there's a lot of good information to be found here if you can filter out all the noise. This is where I learned pretty much everything I know about making knives, and I've made some of the best friends of my life here. But Shop Talk is the only forum I spend any time in, and there are other choices elsewhere that cost less than BF does today.
If paid memberships decline here they will likely be made up by those who do pay the higher rate. But possibly it'll be at best a break-even proposition for Spark, with the same amount of traffic - at least in the short term. One thing about the internet, no one has figured out how to lock anyone into anything; as soon as someone thinks they've found a way to secure an enhanced income stream everyone goes off to the newest, greatest free thing. Or in this case, find a less-expensive alternative. But I doubt there will be another forum with as much diversity as this one anywhere for quite a while. To the extent that's valuable, folks will continue to use this service and many will pay the new rates.
Honestly, it might be worth the price for those who sell their knives here. Many makers have built good careers at BF. Seventy five bucks a year is really not much for the kind of exposure a maker can achieve here, if that's what they're paying for. In the end business reality will dictate the charges for services like those we pay for at BF. Other forums that charge less may find that's not workable long term. Those will either fold or increase their charges to a sustainable level. True competition really does work to create the best - and most - alternatives.