So far in my experience watching small to medium sized companies try to use it for real work, it has been occasionally useful for exploring apis, odd bits of knowledge etc, but overall wasted more time than it has saved. I see very few signs of progress.
The time has come for llm users to put up or shut up - if it’s so great, stop telling us and show and use the code it generated on its own.
With that as a metric, 1 Senior + 4 juniors cannot build the company with the scope you are describing.
A 50-eng company might have 1 CTO, 5 staff, 15 Seniors, and 29 juniors. So the proposition is you could cut the company in ~half but would still require the most-expensive aspects of running a company.
Also often it takes a senior dev _more_ time to _explain_ to a junior what needs to be done than it takes to do it himself. What LLMs give us is the ability to generate a feature about as fast as we can type up the instructions we would have, pre-AI, given to a junior dev.