By having a larger pool of players in the first place?
Logically to get a group of 28 people together on any given day you need to have more than 28 possibles. Someone is always going to be unavailable for some reason.
Its a possible and logical conclusion which would also give room for those who are happy to do concerts but not contests.
We borrowed a trombone player as 2 of ours were unavailable and 1 cornet player as 2 of those who were going couldn't. Also don't forget that many lower section bands are finding it difficult to recruit players (which is why its common to see bands in 4th section playing with only 3 front row cornets / 3 basses / 1 euph etc).