Had the same issue with my first startup. 50:50 split, no agreement in place for the eventuality of a founder disagreement or one founder leaving the company. We were able to negotiate an acceptable solution but emotionally it was quite draining and it tarnished a good relationship/friendship. Also, in the end the company was not worth much, so in retrospect much quibble and stress about nothing.

I think the problem is that in the eyes of a founder their company is always worth a lot, as they tend to see the potential instead of the status quo. That said trying to leave a company that isn't profitable while keeping a 50 % stake and offloading the job of getting to profitability to your co-founder is pretty toxic. Asking to retain a small stake depending on the duration of your tenure seems reasonable though. For a bootstrapped company I think a 4-6 year vesting schedule is reasonable, though it highly depends on the situation of the company. Some businesses are still nowhere near profitable or stable after 4 years, so they won't survive one founder leaving with 50 % of the shares as it will make future investment highly unlikely (most VCs would never touch a company with a 50 % shareholder that has already left) and will destroy the morale of the remaining founders (as they will often think that half of their work will go towards someone who no longer contributes in a meaningful way to the company).