Completely disagree (except the Sonnet bit, yes, it's degrading).
"then the cost of executing that plan is a just one turn more of cached tokens, and minimal time."
This is just not true at all. There's a huge gap between 'figured out the hard stuff' and 'rock solid'.
Dependencies, integration, corner cases, docs, testing, unforeseen issues, a lot of back and forth auditing making sure things are really tight.
Audits get diminishing marginal returns, but you have to do them until they don't find anything, and that's usually a few cycles.
So aside from the fact there is 'a lot of labour' - part of the plan (maybe the most important part) is documenting most of the trip-up scenarios. If you ran an experiment or two in the background your agent will 'discover' a few key odd things, you back those into the plan.
I'm 100% certain that this pattern works because I (and others) use it very successfully.
Hint: save your main context by using sub-agents to do grunt work - even in impl phase - farm out anything directly implementable without a ton of background.
Also - make a skill so your Claude can call Codex and visa versa and maintain long-running sub agents of 'the other kind'.
An Opus with 1M context window executing on a 'plan' that a Codex 'sub-agent' is executing on - ad a different Opus sug-agent is auditing hard ... that 1M token window is dramatically extended to 'many millions of tokens'.
That can work within Anthropic/Codex Pro plans.