Presumably AmEx had a pretty good (though not perfect) idea of what this would do before they decided on the policy. Everything is tracked so it should have been easy enough to know how many guests this was likely to either eliminate or monetize.

The only edge cases are when a primary cardholder brings in a guest who is actually also an AU, in which case the could still get in.

Crowds at the lounges on/near the weekends mostly seem to be families and groups; I can't remember the last time I was in a CL and it was crowded with individuals. Maybe some of those guests are (or soon will be) AUs, maybe some will start paying the guest fee. That all seems net positive for everyone.

I've been in the senator lounges at FRA a number of times during "business traveler" times of days/weeks where it was busy and nearly everyone was a singleton, but that is pretty rare and usually moves quickly since most of those folks are just there for a short time, not yuck it up with friends for half a day.