We really do need to get away from this idea that charities using volunteers=wonderful. I'll declare my interest now by saying I have volunteered for a charity before and I still do some on an ad hoc basis, and I think it's something that can be very positive. But it isn't always better to have someone in a voluntary capacity.
The thing with volunteers is that we're there to do what we want to do. I've done a lot more of the stuff nobody likes doing when I got paid for it than I did when I've been salaried. Salaries make sure stuff that needs doing gets done.
There's also the issue of commitment. I have come across some very committed volunteers in my time, but the reality is that there aren't many people who are able to make it as important a commitment as it would be if it's what pays the bills. Not many people are in a situation where they have the freedom to prioritise it over earning income and/or caring responsibilities and/or complying with the requirements of the JobCentre.
If you're able to get one of those people, that's great. Quite often though what happens is that people are (perfectly understandably) using it as a bridge back to work or into a career or whatever, and once they're trained up they then take their experience to someone that can pay them for it. Which I don't criticise them for at all, because people have to eat. But that then has an impact on the organisation. This isn't such a problem when the volunteer was doing something relatively unskilled that most people could do with minimal training, like stuffing envelopes or whatever, but it can be when they were engaged in more niche work.
And funders don't tend to like volunteers being in key roles either, probably for the reasons I've just mentioned. It's one thing if it's the Thursday afternoon receptionist, quite another if it's the person responsible for strategic planning.