OS + linux internals + reliable systems = 3 non trivial skills, harder to find and more expensive. The combination is definitely rare.
> Breaking out the roles and trying to hire for all of them helps ensure you don't get blindspots.
If you break out the skills and hire for them independently. You'll end up with a lot of people with the common skills and barely anyone with the rarer skills and the ability to train them to others.
I agree with getting people with different strengths together. It's a NP-complete hiring problem though.
You can't fix unrealistic expectations.
I don't think you want to hire someone for an SRE role who doesn't understand algorthims well enough to do detailed analysis of the codebase, including, say, rudimentary big-O analysis of the code. Good SREs read quite a lot of code and if they're debugging a performance issue and the code contains portions that are obviously exponential or factorial in time, you want them to be able to see and recognize that quickly as a potential cause of the problem being debugged. The same is true of problems caused by bugs or anything else. Likewise, good understanding of what's possible and not possible with software is necessary for area to file (or potentially close) reasonable PRs for the codebase.
The person who thinks in terms of page faults and systems analysis may take a little more time to find the optimal algorithm for an abstract problem than someone who only thinks of abstract problems. But they should know enough about software and computers to recognize the limits of their knowledge, and that's the hard part.
The distinction is practically useful because not everyone is an expert on everything, and if you focus your hiring around very narrow markers- e.g. ability to work with algorithms to solve a coding interview- you may not build an organization that has balanced expertise.
Breaking out the roles and trying to hire for all of them helps ensure you don't get blindspots. The software engineer who spent lots of time thinking about operating systems, Linux internals, and how to build reliable systems out of unreliable parts bring value to the organization in the same way that engineers who focused on mastering algorithms bring value. Everyone needs to be strong in everything, but in practice we have different strengths, and getting people with different strengths together seems to be a good idea.