The impression I get is that people either decide to run a full node on purpose or just go straight for a SPV wallet. Running a "semi-full" node (eg. Bitcoin Core with pruning enabled) seems to be the exception. Accordingly I doubt that providing the ability to run a semi-full node increases the overall node count much. However I'm just extrapolating from anecdotal observations without having anything substantial to back this claim up, so don't take my word for it.

I think the problem at hand is, that the fewer full nodes there are, the more traffic they need to bear. This in turn will make running a full node even harder, causing more full nodes to drop off, further increasing the traffic on the remaining nodes until only a handful of very costly full nodes are left. And every new pruned node that comes online needs these full nodes to bootstrap, lest they won't even become a semi-full node.