But there's no way to collect without engaging in some promotion: placing the Flattr button where it's visible and talking it up.
And the first wave of signups won't be sites that have the subscriber-pull to add their own paywalls. It's the (much larger) group whose only current option is advertising. These sites have a more simple calculus: can't possibly hurt, might help, might create some warm fuzzies if they like the concept.
It's after that group signs up -- spreads the knowledge -- that nudges to signup could be deployed. These probably aren't paywalls; they're more likely payspeedbumps.
Contenture had a weak name, weak explanatory copy ("freemium for the masses"?), and weak marketing/PR. Its automatic subscription-divvying process would tend more to reward collecting without promoting. And, they quit fairly early.
The key for any such system to work is awareness, adoption, and critical mass -- so it's not reasonable to take the example of a different, nearly-stillborn alternative and suggest it reflects negatively on another variant that's already acheived more attention.