Paraphrasing your closing paragraph: “you may not use this software if you have not already contributed to it” (because enforcing a future is hard).

Seems a bit contradictory to the license terms. Gonna be a trick to get legislation to enforce something that isn’t in established contractual language.

Edit: on the positive incentive approach, getting double-funded with discretionary budget for every FOSS developer they hire might be a viable strategy.