that's a pointless endeavor. The seniors should just be building everything, not wasting time working on a harness that won't guarantee results anyway.
The seniors shouldn't be building the product; the product team can build the product directly. There is no reason for a product manager to ask a senior engineer to fix the spacing or update the tab sequence for some workflow. The harness should be sufficient for them to do at least that. Get it right and a product manager should be able to build whole features if enough of the heuristics and system level constraints are in place.
Then there is no reason to have engineers at all.
Not quite; the point of the engineers is to build the machinery to make this "safe". Establishing key architectural decisions, rails, and safeguards; things that a product team isn't going to be able to resolve on their own.
What it does speak to me is that only the most senior engineers still have a significant role.