About 5 years ago we hit the same issue. 3 of us set up a Band and we bought the PA jointly ( actually I paid for it all and the others paid back their share from gig earnings over time).
After about 3 years the singer left amicably and I asked her if how she wanted her inital stake back (it was around £180). She replied that without the PA she would have not been able to earn her gig fees and she'd had her inital stake back many times over, so she was fine with that.
A year later the guitarist left amicably and whilst he did not ask for his stake back, he used it as a lever to get me to do the PA for his works function, although I got paid on top as well. In the meantime I'd upgraded the PA at my own expense, using the money received for the old one to offset some of the new ones cost.
I regard the PA as "mine" now and seeing as I'm now 300 miles from him, he's welcome to try and get back his stake! I should also mention that I sourced and organised all our gigs and many occasions, unbeknown to him took no or reduced money in order to keep us going. My shortfall was more than his inital stake for the PA too.
I do like the idea of a shared stake which if someone leaves, the new member pays into, however with depreciation how do you work it out?