I wish the airlines would do differentiated flights (maybe adult-only or 12+ only for one flight a day on otherwise high volume route, and maybe an explicitly pet-friendly flight on a route, etc.). I live on an island, and especially now that most airlines (covid scheduling issues) have eliminated animals as checked baggage, anyone with an animal which doesn't conform to normal cabin transport is kind of screwed -- they have to use a specialty transport company, which either flies on dedicated cargo flights or something else, or they charter a plane ($5k minimum for San Juan to Miami).

My personal preference would be zero children, zero dogs, only well behaved cats, but I realize that's a niche flight preference, but on a route with hundreds of flights a week, one flight a week (or even one a month) should be easy to fill.