My girlfriend and I have had this discussion a lot, because she went to a top tier art school. I think you might be underrating three factors in your reasoning:
One is the value of very direct feedback and guidance from seasoned, respected professionals. The class sizes in top art programs are very small, and most classes have a frequent, intense “critique” portion where the professor and students tear apart each others’ work. It’s like 3 years of intense code review from other talented programmers, led by a staff engineer. It’s hard to put a sticker price on this, because you largely get out what you put in, but it’s immense and I’d argue unique to a top tier art school.
The second is that you’re living in a high pressure environment of professors and peers who always seem to be pushing you to be better in some dimension. It’s not conducive for everyone, but if you have a strong competitive drive, it can make you very productive.
Finally, the fundamentals in design and art are insanely important. My girlfriend took 4 semesters of typography, because the ROI there is legitimately defensible. Most people skip this stuff when they’re self teaching, and it shows to people who know better.
You’re right that portfolios are of paramount importance, but good companies will make you defend your portfolio pieces, and ask you what you’d do differently with different constraints or resources. An art school training will humble you to the idea that there is another way, and arm you with ideas for alternative solutions. It’s subtle, but it comes out in interviews and on the job. I’d argue it’s similar to software engineering in that way.