You are right about guaranteeing the future, nobody can. However a case could be made to draw build assurances into an agreement that takes into account downsizing etc. Speaking from my slightly left of centre viewpoint, you can write what you want, but SOSR overrides it anyway.
Good evening Litebulbs.
Although no company does guarantee anyone's T&C's, BA actually did originally. In the initial negotiations (term used loosely) we were offered a Monthly Travel Payment MTP to replace existing allowances. It was even calculated on the year 2008/9 (high profit) allowances.
BASSA said no and walked out the room.
Had we had the MTP last year, with ash cloud, snow disruption and now this year with all the flight disruption due to political unrest/natural disasters we would have undoubtedly been quids in.
To see Miss M demanding a guarantee now, whilst still funding the union that threw it away, makes me weep.