We did a user research study measuring website breakage under various privacy protections:
tl;dr - strict-origin-when-cross-origin was one of the protections with the lowest amount of breakage. Entering Private Browsing is a clear, strong signal that the user wants more privacy, so we started by implementing this protection in Private Browsing.
However, note that some advertisers demand that AdTech vendors must not serve their ads on certain kinds of pages. (e.g., https://support.google.com/adsense/answer/1348688?hl=en&topi...) Many of those agreements require full referrers to be able to audit the ad inventory.
So there are some concerns and trade-offs to make in this space.
If Mozilla's users are saying “yes, this is good; please do more”, Mozilla can use that as a defence against any resistance from advertisers.