Despite paying it a lot of lip service, in practice companies demonstrate that they don't really care about security that much, so I don't think that resistance to prompt injection is the main blocker for any products right now; if some product was very useful but had issues with prompt injection, then people would complain a lot about it while still using it and paying for it; and security (much less the even more niche requirement of provable security) would only be a differentiator among multiple competitors providing a comparable product, not a barrier for selling the product in the first place.