I think a long term dev fund is actually really bad because eventually Zcash will need to ossify if it really wants to get above and beyond $1 trillion. In that world of soft-forks-only, the dev fund honestly becomes extractive and creates an overly powerful dead weight foundation/labs team
Up until the point of ossification, I have more mixed opinions about the validity of a dev fund. The main problem honestly is how you align incentives to get the dev fund turned off eventually, which i believe is essential