This issue seems overblown. Sure, if you apply pure GPT-4 (or whatever) to a summarisation task, it will cause the problems mentioned. But you can have another AI that previews content first, looking for prompt injections - and only when the content is deemed safe (or sanitised) it gets forwarded to GPT-4.

It's one thing to produce a prompt injection, but another thing to produce prompt injection that avoids detection by multiple layers of such analysers.

Similar multi-layer systems are already being used, with success, for sanitising outputs from various LLM and diffusion models.