I've always found that to be a perverse incentive model. I don't know how to do any better though. Maybe shaming companies that use OS projects heavily into donating? Not really sure how to scale that either.
I am sure they considered it and figured it wasn't a good model for them. It is unfortunate that I have (with much pleasure) used rethinkdb in several projects, yet haven't had a chance to pay them for a service that would be useful to me.
As far as I can tell the codebase is designed to be as opaque and complex as possible. Any normal organisation will pony up the fees (between $1000 and $5000 per annum - https://cms.orckestra.com/Products/Plans) rather than try to really get their head around it.