Is there evidence for it not remaining a strong signal?
Once you have work experience, either from your first few jobs, or for some from internships, then your school rarely matters. It's not like you learned some revolutionary thing that is exclusive to MIT 20 years ago, or that whatever you learned is even applicable for that matter.
They take these two week courses at these universities. And then they put it at the top of the "education" section of their resume.
It's like $99.99 prices.
Against all human logic, people just cannot look past the magic words "Harvard University." And people aren't dumb, they know it's a two week, meaningless course.
Doesn't matter. Lizard brain sees Harvard.
... then your gpa rarely matters. But your school always matters. Not only in terms of network, but in terms of reputation, prestige and even camaraderie. Especially the higher you go up the ladder when more and more people tend to be graduates of top schools.
Nobody will ask about your GPA as you get further along in your career. But everyone will ask about your alma mater. You eventually stop putting your GPA on your resume, even if you had a 4.0. But you always put the college you graduated from.