If a sports photographer is using the R5 as the main camera and the R3 as the backup, they would be idiots...so it's not going to happen. So when you say "merely a backup" - you are making what is almost certainly an incorrect assumption.Flagship camera price for a 24mp non flagship device thats merely a backup to the R5? At least the Sony A1 was released at 50mp even with EVF blackouts. The R5 is fast enough to do what is needed for sporting events. Jeff Cable proved that in Tokyo.
Jeff Cable used the R3 for most of his shots at the Olympics, by the way.