It's kinda the opposite actually. iframes didn't provide sufficient security to do the sort of things Google wanted to be able to do with them, so they had to design a new standard with better protections: https://github.com/WICG/portals/blob/master/explainer.md#why...
I don't understand the cynicism people have for this idea. All they did was say "wouldn't it be cool if you could have nice animations in between pages" and built a proof-of-concept. It's not a finished product. They aren't forcing it into a standard. It's a demo of something that would be cool.
My point is, portal isn't "an iframe without all the security protections." portal is a demo of animating between pages. What it becomes from there is completely flexible.
Things are not "flexible" once they have been shipped on by default, typically. Changing behavior or removing at that point becomes very hard, requiring usage measurements, etc.
when you are google, unilaterally releasing and pushing a major new feature for “the web” has an entirely different meaning and implication to it compared to, sadly, mozilla, or some other player (even apple to some extent) because of their huge market/mind share.
in that scenario “wouldn’t it be cool” is not a good enough reason, and for a major feature such as this, skepticism is healthy and warranted... the “web browser” is slowly being transformed into “the google browser” and we have no one to blame but ourselves
The consensus opinion seems to be, from this thread and elsewhere, "While Google is not doing anything wrong by standards in this case, because they have the power/potential to do something wrong by standards we must oppose this as well."