The truth, however, is that a senior developer is $140,000 and a junior developer is $80,000, and that same junior developer will be $100,000 in two years. (This is assuming salaries in one market.) That junior developer will take 3-4x the time to handle the same task as a senior engineer, and will take some of the senior engineer's time to guide the solution, provide feedback, and mentor.
That doesn't mean that we shouldn't hire junior developers - in fact, it's our duty as an industry to hire and mentor. However, it is with open eyes knowing that junior developers are not cost efficient in many areas for the first year.
But we go back to the original statement: "With all due respect, if hiring an entry level dev is a loss for your business then perhaps don't hire them? A dev should be a net gain regardless of skill level."
If you have $320,000, and can have 3 junior developers or two senior developers with the same budget, the 3 junior developers will be a net loss for the first year. They can be productive, but you have to compare on budget.
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But I find that as a bizarre set of work that you call out.
* Refactor work is often the largest need from senior developers. Learning how to refactor safely is a separate skill that is not a skill junior developers have without instruction and feedback.
* My senior developers write tests as they develop. If there is test debt, it means sitting down with a copy of Michael Feathers and that is definitely not an introductory text. Trying to write tests for code that wasn't written with unit tests in mind requires a set of skills that many senior developers don't have, much less junior developers.
* I'm curious what processes you see that junior developers can improve.
* Researching tasks can be something useful, but what are these tasks that aren't done while a senior developer needs a break?
* Low hanging fruit bugs are likely the most obvious thing for junior developers, but a highly cohesive team that is writing tests has fewer bugs that are low hanging fruit.
* What systems can a junior developer blueprint? A senior developer understands patterns and quickly solves the problem.