Having decided to become a researcher after long career in industry, should I work independently or start a PhD?
A PhD is training for research.  There's a whole lot of things that happen along the way, but the point of the process is that you gain skills in conducting research.  In addition, you gain the 'worldview of a researcher': how to think critically, assimilate information, analyse data, collaborate and come up with new ideas.

It is, of course, possible to do all these things without doing a PhD.  But the advantage of the PhD is it gives you opportunities that you might not otherwise have: access to resources, specific programmes (eg internships or field studies), people to collaborate with, exposure to new ideas.  Those opportunities can be open to PhD students in a way that they aren't to random members of the public.

A big part of research is interacting with other people, not just working on your own single path.  PhDs force much more interaction than you might do on your own (though it varies by subject).

The other side of 'training for research' is that, once you've come out of the process, you have simple proof that you have been trained.  That's what the letters 'PhD' are code for.  While the experience can be very different at different institutions, a PhD means the holder has untaken a major research effort and it has been validated by their peers, and learned some of the skills of research along the way.

That's the reason that many academic jobs expect you to have a PhD: they don't have time for a free-form assessment of your research skills - the PhD is proof that somebody else has already done that assessment.

By doing research without a PhD, you force people you interact with to do more work to assess your skills.  You can do that by building up a good body of peer reviewed research, which itself is unlikely to care whether you have a PhD (especially if the review is blind).  But in superficial contexts (eg screening for a job, a grant application or an invitation to some event) you're likely to find yourself at a disadvantage.

One advantage with working inside academia is you have more collaborators, but also as you become more senior you can build a research team: when students work for you, you (collectively) can achieve more.  That's difficult to do when you're working independently.

Finally, we can all come up with examples of brilliant people who started working on their subject without a PhD.  I suspect that's much less common than it was in the twentieth century, for two reasons:


Supply and demand: there were many fewer researchers, and the process was much more informal.  Now every tenure job gets hundreds of applicants. Why would you pick one missing critical training?
War and social upset: when you have to work on radar/antibiotics/aeronautics/nuclear physics/intelligence/cryptography at short notice for the war effort, all employment norms go out the window.  When those people came back to civilian life, they already had skills they could use in research.  Or, alternatively, they proved their worth in opportunities available because everyone who would normally take them was away fighting.


(Side note: I come from the UK where PhDs are 3 years on paper and often more like 4 in practice.  The North American process seems like a much more drawn out affair, and the time tradeoffs may be different)