On the point about volunteers being replaceable or not, IMO healthy projects have a good mix between full-time contributors who are very often paid (otherwise they couldn’t do it full-time), casual volunteers (e.g., who might submit a bug report now and then), and part-time volunteers who do a lot of critical project work on an ongoing basis. I think it is this third group, the committed part-time volunteers, who are most irreplaceable; as you point out, they are typically both technically capable and highly knowlegeable in how the project works and how to get things done. Full-time paid contributors do useful work, but sometimes they are not as plugged into the project and its ideals as committed volunteers.

A project can always go out and hire someone (if it has the money) and can also try to attract more casual colunteers to replace old ones, but it can sometimes takes years to recruit committed volunteers and have them evolve into key contributors as they gain experience and standing within the project.

On the subject of divergent ideas: You are right, conflicts over priorities, goals, and project ideals in general can “turn off” contributors; as you imply, this can especially be the case for committed volunteers who are participating primarily because they believe in the project and not because they’re being paid. (Thus your two points are closely connected.)

I agree that there is no “magic formula” to address problems due to divergent ideas. I think the best one can do is to try to treat project contributors and their views with respect and work to ensure that contributors are involved in project discussions and decisions that might affect them. At least that way any conflicts can be handled openly and honestly, as opposed to key contributors feeling that their opinions have been neglected and denigrated.