Doesn't ASLEF think that it is all right for the drivers to "open the doors" (as if that were the guards' entire job) on Thameslink?
No, ASLEF does no think that at all.
It tried to stop that, too. It is just that it might succeed this time.
Remember, ASLEF is the drivers. It is not protecting its members' jobs here.
It is protecting their lives, and everyone else's.
And it is going to win.
This has already become a matter of "both sides are as bad as each other".
As soon as that is the general opinion, then the union knows that it is going to win eventually.