The 3s and the 4s are supposed to be going to Streetlites, for comanlity. Streetlites do appear on the 5 on occasion. Neither can I see deckers going into either. Though I have seen one on both. Years ago there used to be a 3 on a Saturday morning from Mead Vale to Town which was a decker and weeknights when I was at school there was a 5B from the old Sainsbury's stop around 15:30 to Town which was always a Lympalong which then formed the 16:30 X1.
The 3s and 5s won't get new buses they are not lucrative enough, the last time they had me buses was in 1991/1992 with the original Merc minibuses. If any of the Weston Town services were to get new buses, it would be the 7s. I know the desire is there for E400s to replace the B7TLs and I know the Streetdecks haven't found favour in this area, but they would be perfect pootling around on the 7. Everything they are used on in this area is hilly, the 70/71 round Ashley Down, 90 going from St John's Lane up into Knowle and the 376 between Bristol and Wells. It is said they cope well in Norfolk which is relatively flat, the 7 is also flat bar one slight incline as you come out of the Coronation onto the foot of the railway bridge on the Broadway.