@IamnotwhouthinkIam I don’t understand why John dipped in popularity
Jack replaced John.
There were lots of Johns at (Welsh) school with me, but other forms of John were becoming popular. The English names tended to be replaced by the Welsh versions, Siôn and Iwan, then Ifan, Efan, Ioan and Jac.
Other form of John like Seán and Ian were fashionable.