Limiting access is a bad idea. If with even the three hour limits and no arrivals they are turning people away they should expand the lounge or open a second location. At many airports that would actually be a third location since many delta members are using those lounges anyways.

DL lounges have overcrowding issues but nothing as constantly bad as what you hear about Amex lounges. They need to solve the problem. They are building their brand around lounges.