We see browsers restricting cross-origin everything more and more, take for example Firefox's recent efforts to segment cookies, or how browser caches are now segmented instead of global.
I agree with GP, I can certainly see a future where browsers make a move to block cross-origin JavaScript files.
I agree that browsers are getting more and more aggressive about blocking external content. I am all for good privacy protections and I understand the reasons why they are doing it.
But as a user, I am starting to get frustrated by how often I visit "normal" sites that just don't work in certain browsers because of all this blocking. There are plenty of legitimate reasons for people to load external content on websites, and it's important not to throw the baby out with the bath water here.
I am also concerned that certain big tech companies, notably including Google though it is hardly the only one, have a tendency to shoot first and not even ask questions later in terms of collateral damage when they deem something to be the appropriate course of action. Chrome has historically had no problem with killing off major functionality that some useful sites required, and other browsers have often followed. Apple has a tendency to do the same (or to achieve the same result by refusing to support functionality in the first place) particularly on iOS. It's always done in the name of improving privacy or security or reliability or some other worthy cause, but it still effectively removes useful content from the Web based on the decision of a handful of people who work on browsers. We should be very wary of that kind of power, particularly when it is wielded by people with little accountability or oversight.
I agree with GP, I can certainly see a future where browsers make a move to block cross-origin JavaScript files.