The effort it would take us to truly differentiate in this space is not one we could justify
I disagree. I think the amount of effort Stack Overflow would need to differentiate in this space is zero.
It's not about the tech, it's about the types of people that use it. My experience with the SO job boards has been that of much higher quality, more relevant jobs than LinkedIn or Indeed. I don't know what it's like from the other side, but I get hundreds of irrelevent messages on LinkedIn vs a handful of relevant messages here.
Sad times.
Edit: A couple of other thoughts following comments.
I'll add a quote from the "Why use jobs" page:
We're as committed to putting you in control of your job search as we are to helping you get answers to your programming questions
That page lists a rule against most of the problems I have with a site like LinkedIn. I think this is the thing that separates SO to me, you build a community around solving coding problems, and provide a selection of curated additional tools, while guarding that community against spam recruiter messages. That the site is able to do that sets it apart from my perspective, and shows much more respect for the time of its users than other companies. It's a shame that other job boards don't have something similar.
I also understand that there's a cost to maintaining a job portal that may not see a great amount of use, which is hard to justify for a company that provides a free product but needs to pay wages.
I almost want a site with a job portal that doesn't get used much. It means users are choosing to engage when they're looking to move. I think there could be a pipeline that feeds into this, say using the current salary tool to then highlight other jobs, but understand this takes development time and has a maintenance/operation cost, which may not be easy to justify.
I think SO does a great job supporting its community, although I know that's not always a smooth or easy process.