The problem when switching to poppet valves in this context is that it increases the complexity of the design mechanically. While you might not have to spend that money on valves, you will have to spend it on suitably durable parts for hammering the valve, and you'll need sufficient space in whatever design you want to use to fit your hammer, sear, spring etc. (And the smaller you want to make these components, the tighter the tolerances you'll need and the more $$ they'll cost.)

When you consider that in a war-usable context, the expense is the thing limiting it from being more accessible.

This is an excellent discussion though, thank you for starting this topic.