The "international 5" age bracket (I agree the name is awful, by the way) fills a need for many gyms. While there are some gyms who could fill an entire team with athletes over 18, there are many more who have just a few athletes that age who still want to have the opportunity to cheer. Whether it is a small gym who is struggling to put together enough 12-18 year old athletes to form a single L5 team - a medium gym that has enough for a single senior 5, but not really 2 - or a mega-gym that has enough for 4 and not really 5, the "international" age bracket helps those programs find teams for those people.
By eliminating that age bracket option, you would force many of those athletes to either not cheer, or quit their current gym and go to another gym (probably a gym like yours or mine) that has the resources to create entire team(s) using only 19+ year-olds. While that may be great in the short-term for the mega-gyms, that would be bad for the industry, IMO. The state of the economy is such that we shouldn't be making it that much more difficult for gyms to form teams.
To me, it is much like suggesting that we restrict the mini division to ONLY 6-7 year olds, the youth to 8-9, junior 10-13, and senior 14-18. There would be just as many total divisions, but gyms would have a dramatically tougher time making teams without the overlapping age ranges. By doing that, you would have: 1. Cut back on the options for each individual athlete AND 2. Made creating teams more difficult for gyms - all without even the benefit of eliminating any divisions.