Well, if you could create software that is provably secure against at least a large class of cyberattacks, you could also make quite some money, since I know that at least the finance industry would be willing to pay a lot of money for it (source: in the past, I had an opportunity to talk someone who is very knowledgable about software in the finance sector, and I asked him some questions about exactly this topic).
Thus: we have not solved the "cyberinjection" problem for completely conventional software for decades. So I don't believe that we suddenly have the capabilities to solve this problem for LLMs.
But: As I pointed out, people have come up with ideas that make cyberattacks much harder. Here, I can imagine that their ideas could be used to make prompt injections a lot more complicated.
Additionally, we know from decades of experience that making insecure software secure is nigh impossible. Instead, one has to architect around security properties from beginning. The same does, in my opinion, also hold for LLMs and prompt injection.
Here's an example of an agents company that pivoted partly because they couldn't figure out how to secure them: https://twitter.com/zachtratar/status/1694024240880861571
I don't think the lack of a solution is because it's too expensive. It's because we don't know how to do it yet.