In my area we are outnumbered 14 to 1 (Literally) Non-Catholic to Catholic Churches. Our population is small but dedicated. Almost every single boy has volunteered but we were still short. We literally had no choice but to allow girls as altar servers.
That sounds like prevalent contraception in your area.
Keep the boys — that’s how the pathway to the priesthood is formed.
(1) grow more boys, and
(2) recruit upward in age.
There is nothing wrong with having altar servers in their teens and twenties. It gives the service of the altar a vastly increased status: it gives the younger boys something to look up to.
In our parish, on those occasions when no boys are present, the priest serves alone. That includes all aspects of the liturgy, including the censer and incense boat which are repeatedly used throughout every mass.
Why default to girl servers? If there aren't enough boys, recruit men. Might help foster delayed vocations.