The people I know who work in or with admissions (consultants who talk to lots of admissions folks) pretty much all say that SSAT scores do not provide a lot of useful information to them given all the otherparts of the application (and interview)… They can, in general, pick the students who will do well at their school based on other things (including a writing sample done at the interview, for example. )
At the same time, testing is an obstacle for many applicants, and not just in covid times! It is especially so among students from areas/communities where few apply to prep schools (day or BS), not just in access but also in prep.
Being TO makes it easier to apply for students who would probably have good scores with good prep (but lack access or prep). It also makes it easier for a weaker student who simply cannot produce good test scores to apply. Admissions must try to figure out which is which but I suspect that many of the “weaker” students are recognized by the AOs as that.
In my mind, TO increases the pool SOME by bringing in really good candidates for whom getting to or prepping for tests was an issue. (Think of this as access.) This does make the schools more selective. It also increases the pool by a lot of students who are quickly identified by AOs to not have a shot at all. These make the schools more selective in terms of numbers but less so in reality. (I could go to an open audition for a Broadway show, but I’d be a threat to nobody!)
I hope that to the extent there are more people interested in BS going forward that it has the effect of creating awareness of the many excellent options out there, much as it has in the college world. There are lots of schools that offer an amazing education and experience that are less well known. Hopefully, they will be the beneficiaries of some of this interest as well. Not getting into Exeter doesn’t mean you can’t go to a great BS any more than not getting into Harvard means you need to go to community college.