Of course another (temporary) option to fill gaps between the last Air Eng retiring and the last aircraft requiring an Air Eng retiring is to place pilots in the role.
I know, I know, people will kick and scream, but the reality is that pilots are capable of the job. Im not talking about training pilots specifically for the role, just using them as a stop gap.
It worked well for the airlines for many years, with pilots often starting out their career in the third seat before upgrading to the right hand seat.