Lounge access policies

I have heard that there is a big difference in the way European airlines grant lounge access and the way airport longes operate in the US.

Namely in Europe, usually one can get into a lounge if holding a ticket for a flight departing the same day, with appropriate ticket class and airline. Or sometimes when arriving, in appropriate cabin class and airline again.

Whereas the people in US supposedly pay for lounge access passes, or it is a perk of the frequent flier program.

It would seem to me that lounges are much easier to build under the European access rules. The airline can just check the seatmaps of their planes, count the seats in the front end, then add up the departing planes from their timetable – and they know the maximum number of people who can be entitled to access on any day, so they know how big lounges must be built.

Whereas in US, you never know how many frequent fliers or lounge members choose to buy Coach tickets to a single flight.

What are the most popular lounge access rules now (That is, in which direction are they being changed)? Are the airlines trying to add to people who can enter lounges, or are they trying to prevent overcrowding of lounges?