None proves it, but the question was how to break in to the field, this implies an interest in being successful in the field, not cheating your way to a diploma. There are many STEM careers that are hard or impossible to get in to without going back to school and getting the specialized degree. EE, CompE, Chemical engineering are all tough to break in to on your own. You could of course read a lot of books, but it's probably tough and you're unlikely able to get the kind of focus you need on the areas you will use on your specialization. A dirty secret is perhaps that if you go to school for say EE and you study RF, you may have a very hard time breaking in to computer engineering later, or even the much more closely related power systems. These areas end up being super specialized and your school+work experience ends up binning you into your niche. Later in life your "experience" is expected to be pretty fine grained and deep, so the breadth that's frequently touted as an advantage for liberal arts is actually a drawback for STEM fields outside of academia.
The trick is getting past the resume screen, that's tough to do without the degree in the first place. Then once you do, prepare for an 8 hour long interview that is going to make many PhD defenses seem trivial. There's no risk taking in the hiring process, they will be looking for you to demonstrate a large set of active knowledge on the spot. If you make it through there, be prepared to be learning hte rest of your life and never let yourself get comfortable. The fields change fast, your niche may disappear or more likely be outsourced, so you want to be able to shift focus believably and keep your eyes on trends.
In theory some software programming jobs are easier and don't necessarily require a degree, but I would absolutely be prepared to demonstrate expert knowledge on the language they use (exclusively C in my line of work) and if you're in to systems programming you better know hardware really well too. A CS degree really only helps with the social factor, even if you already know how to code well and have some documented experience on open source.
None of what I said sounds like it should be the case, but it absolutely is. It strikes me that degrees are being used as professional training program (ex. med school, law school) not for general education, as they should be.