Excuse the multi-quoting
Another heat of the 1500 adds 5 minutes to the meet. Letting in people who squeaked into the standard right at the end and will more than likely run 2:30+ on the day adds how much time? Regardless, the point isn't that adding more runners is impractical, otherwise why not ask for the Olympic Marathon to be open to everyone who hits that A standard? The actual point is that when an competition is set up to determine who will represent the country at the Olympics, only those with an actual chance to represent the country at the Olympics should be competing.
And yet the rest of USA Championships for running gets near equal participation on non-Olympic years, with much more stringent standards, despite reduced coverage and the Olympic games not being at stake. You or someone else already mentioned how big of a 'celebration' club nationals is, yet that is a yearly event, much like the completely maligned Marathon Championships.
What exactly is the motivation to improve as a 2:22 marathoner when the standard is 2:19? Since a 2:22'er is unlikely to ever become 'elite', it seems to me the motivation is to either get better to see how good you can be, or its to hit that specific time standard so you can attach your name to the Olympics without achieving a level of performance everyone in every other event has to. If we want to encourage running as a lifestyle, we need to get off the model of the Olympics, since that event, and its trials makes running a first-rate sport for 1 month every 4 years. Put a 'soft' standard like 2:30 on the Marathon Championships and make that into an event that gets coverage on a yearly basis if you want to make running a lifestyle for more people in the country.
the 20:00 5k'er is a complete straw man. I said a winner or runner up at a major regional marathon like LA, Marine Corp, Houston, Philly, etc. Who gets more attention for the sport, those people (when they're 'local' and not foreign competitors), or the person who hits the standard in some random marathon? I have a friend who hit the standard in 2016 via a half-marathon time and went on to compete at the trials. He got a blurb in one of our regional running publications, and merely mentioned as part of a list of regional people competing there in our other regional publication. The 2 'mid-major' marathons in our region both get multi-page spreads of coverage with in depth interviews by the top-2 finishers (or the whole podium if its a close year).
If you add to the standard, and we go from ~150(?) people qualifying to say 300, that further diminishes the value of everyone hitting the standard. particularly since many of the new qualifiers won't even be on the podium at these sub-elite events. This goes doubly given that those who understand the sport already know how much 'easier' the Marathon qualifier is than the rest of the Olympic trials.
When an competition is set with the purpose of determining who will represent the country at the Olympics, only those with an actual chance to represent the country at the Olympics should be competing.