RMS is not saying "it is better to stay on full proprietary". But many users have no choice. If he will not allow proprietary bits in a free stack to make that stack usable by blind (or otherwise disabled) users, and if no other free or partially-free choices exist, then those users will be forced to use an entirely-proprietary platform (which very likely is built on Windows). Therefore they are less likely to switch to a free set-up when it becomes available. That is not RMS's stated goal, but it is the practical result of his dogmatism.