As I've explained before there is no 'rush' for Longhaul.
If you join now on the SH fleets then you will have your seniority number and thus your 'place' in the pecking order. If they did start recruiting directly to the LH fleets after a year or two you would sit there on SH feeling peeved until you realised that when you do move across all of those who went DEP before you will be pushed down beneath you for bidding.
The current situation means that those with 'decent' seniority on the Airbus are slotting into the 744/777 lists at over a third of the way up the list and achieving good rosters and decent bids.
Waiting is never a good option with BA.