I don't know a lot about synchronized skating, and what I do know is mostly US specific.
In my area there are two synchronized clubs that each have several different teams at various skill levels. None of them is senior level.
According to the US Figure Skating rulebook, here's some edited information about the ages and number skaters required for each competition level:
Preliminary 8-20 skaters who are 13 years of age and younger
Pre-Juvenile 8-20 skaters who are 17 years of age and younger
Open Juvenile 8-20 skaters who are 19 years of age and younger
Juvenile 12-20 skaters who are 13 years of age and younger
Intermediate 12-20 skaters who are 19 years of age and younger
Novice 12-20 skaters who are 19 years of age and younger
Junior 12-20 skaters who are 12 through 19 years of age (Note for ISU competitions, you must follow ISU requirements)
Senior Elite 12 12 skaters who are 17 years of age or older
Senior 16 skaters who are 17 years of age or older
Most levels also have minimum test requirements.
Then there are also levels for collegiate teams, adult teams, and Special Olympics teams.
So whatever happens at the international elite level is not likely to have much effect on opportunities to participate in synchronized skating at recreational or developmental levels. There's no reason to believe that USFS (or other federations with many clubs hosting teams) would suddenly require all levels to downsize their teams to the lowest number of skaters.
According to the latest (2024) ISU Special Regulations Synchronized Skating,
I.e., at most levels there is some flexibility in the number of skaters on a team, which allows clubs, or federations, to form teams even if they don't have 16 skaters to fill out a team, and to compete internationally with fewer skaters.
However, for ISU championships, 16 skaters are still required.
Here is the ISU press release about the new Synchro 9 category:
The International Skating Union (ISU) is delighted to unveil a groundbreaking development in Synchronized Skating with the launch of “Synchro9”– an innovative competitive category designed to elevate the discipline, expand its global reach, and strengthen its pathway toward an Olympic future...
isu-skating.com
As of now, it does not appear that the intention is to eliminate the 16-skater format, but rather to introduce an additional category that is more likely to be included in the Olympics than the current standard team size.
I suppose it remains to be seen how that will play out if in fact the Synchro 9 category is accepted for Olympic inclusion.
If the ISU continues to offer standard junior and senior championships for 16-member teams in addition to Synchro 9 events, that could make for more opportunities at the elite level. Just not in the Olympics. But it's not in the Olympics now either.
We'll have to wait and see where they want to go with this. So far there's no reason to jump to the conclusion that the sport as it currently exists will be eliminated in favor of the smaller-team version.