I think the HN crowd has lost its objectivity in evaluating 37signals and its become something like an Apple.
One-time forever pricing is a very innovative concept for a web based SAAS application. I bet we will start seeing more of this soon.
We need to remember, in SaaS it is the "service" that you pay for not just the "software".
It is okay to be skeptical when someone says, I will support you forever when there is an ongoing cost somewhere.
One-time forever pricing for a service which costs you ongoing money to run is suicide unless you're subsidizing it somehow.
You have to have a subscription Basecamp account to buy the $25 one-off thing. So you're essentially paying a fee to hide projects that you don't want in your company Basecamp.
This is essentially a free tier with a one off filter.