I think the 50/50 profit arrangement sounds good, but your co-founder probably knows its not built to last (not as future proof as equity is). The key thing wi

I think the 50/50 profit arrangement sounds good, but your co-founder probably knows its not built to last (not as future proof as equity is). The key thing with the incentives is whatever s/he may be getting now is less than his/her expectation of future value of other things s/he may be working on. If s/he needs to work, and has nothing saved to live off, s/he will value a certain piece of cash component more than stock today and will prioritise the bare minimum first before your project with him/her. If your co-founder is neglecting duties currently, it's likely s/he is not getting enough today. If the company does well in the future naturally s/he will work better in the future when the company performs better, this is what your arrangement incentivises him/her to do.