1 or 2 "senior-level" folks (imho, people with at least a decade of experience within a multitude of environments) are going to outperform a half-dozen junior engineers.
When you're building a monolith Rails application, anything goes. But now that the industry is trending towards (for better or worse) service oriented architecture, stateless cloud deployments, walled gardens tossing data back and forth over RPC, data science teams, etc... you need people who have been to war a dozen times and have already made all the mistakes. That is what you get with a senior engineer: someone who knows what NOT to do.
I have spent most of the last 2-3 years of my career playing bad cop and telling people no. Shutting down wild and exciting ideas. Stopping over-engineering in its tracks. Preventing an investment in a bad technology. Preventing fragmented infrastructure with data in a handful of different databases.
I think that is why the junior engineer has disappeared. Most of us were never trained/educated/encouraged to be good teachers and we spend most of our time putting out fires and trying to keep our hands on the pulse of technology while simultaneously hanging on for dear life within our organizations. Pivots, layoffs, blockchains, shiny new business objectives are all mortars flying at the eng team left and right.
I wish I had junior engineers to mentor and teach and foster into folks who have good critical thinking skills and I can trust 100% to make the right decisions – but there is a tremendous cost involved to the business that I think most are unable to afford. Some of my most rewarding career experiences involved working side-by-side with folks fresh out of a bootcamp or university who were wide-eyed and bushy tailed. It was a lot of fun watching them run off and do a bunch of work then having a discussion on the choices and using each moment as a stepping stone towards a bigger understanding of all the moving parts in modern infrastructure.