They do not even have to go as far as saying all new commands are on the A320.
K.I.S.S. is the answer. All they need is transparency and a seniority list.
All commands are advertised / awarded on seniority and come with a two year freeze.
If you do not want a A320 command do not bid for it and wait for your number to come up on the A330, A340 or whatever.