So what? You think that the guards deserve longer than 6 years guarantee for their jobs, and that they should strike to get it. Fair enough. They're entitled to do so.
Everyone else has tried to explain that this is a fruitless position. Within 6 years a significant compliment of the guards could be removed from post. If enough new rolling stock were ordered, they probably all could be. This would happen either through dismissal, resignation, redundancy or a new contract for a different role being offered. This process will probably begin soon after the first new trains enter passenger service, a matter of months away, not 6 years.
If the union negotiated a deal for the guards, they would have a legally binding guarantee of what would happen to them. If they do not, as it seems they will not, they will have their job for as long as the company needs them only. How long do you think that will be? For most of them, it will be significantly less than 6 years.
This has been explained enough times now though so I'm afraid I'd you still don't get it that's the end of the matter.