Users should be able to have full control over their experience interacting with third parties if they want it. This isn't unique to post-LLM stacks like this, but it seems like this shifts the balance of power.
The next step after injecting custom UI controls is to build completely alternative frontends. The next step after that should be to build generic local frontends that abstract over multiple comparable thirdparty providers.
I'm solving this from the other side of the equation: we work directly with the SaaS vendors to make vibe coding embedded into their platform. Working with some Series B companies right now, 2000 business users are now able to build any feature they want, within the guardrails of the SaaS vendor. (More info in profile if anyone wants to chat)
Exciting times!