Let's say $2m/year - a sellout now at $40m is 20 years (if we (unrealistically) discount inflation and interest etc.).

You love what you're doing, but who knows what everything's gonna look like in 2-3-4 years, much less 20? Google might decide that DHH is a punk and make improved clones of the entire 37s portfolio and release them for free. 37s may have a breakdown and find out that their backups for the past month are corrupted, and never regain customer confidence. The concept of SAAS/cloud might be compromised by Patriot Acts III-VI. All those are worst-case scenarios, but there are a million other events that could at least compromise the $40m price-point -- so waiting and taking the money later isn't a backup-plan.

If you take your $40m now, you're insured against all that, and also against growing bored with the project and conflicts with co-founders or key employees -- and the buyer just might let you work, even on salary, on the project.

Passion now != passion in 20 years. Actually, I'd venture as far as saying that if you're sure your current passion is going to last, it's not real passion.