The 365s seem to do a perfectly good job right now. Can they last another 20 odd years?
I just echo the other comments here. 365s are a good unit, but have even more potential. I can well believe that with some sensible engineering I think 110 is not only possible but actually a good idea. The Peterborough fasts will be able to clear further north before a 125mph train catches up. At the moment, you have the timetable set so that often you'll have one of the evening fasts leaving 8 or 9 minutes before the following HST or 91. It is usually the St Neots to Huntingdon stretch before that following train has caught up. This is an inconvenient place, because if the following train is onlt 1 to 2 mins late, the 365 ends up on the down slow out of Huntingdon and has to wait at Conington for the HST/91 to pass. With 110 capability, it could be berthed at Peterborough by the time the HST/91 has caught up.
If I was to give the 365s a mid life overhaul, I'd regear/motor them for 110 (assuming infrastructure testing proved positive, and there was budget available), and install air conditioning. For a semi-long distance commuting unit to last another 20 years, I think that is essential. Anyone who has sat on a 365 at King's Cross waiting to depart just sent from Hornsey on a scortching hot August day will know that the ball vents are not up to the job and the windows help a little, but only when moving. When there are delays and the train stops, it's like being on the tube.
I'd also do the requisite re-carpeting and re-covering of seats, or installing new seats if regs required.
But yeah - all in all, they are fantastic trains to commute in and the potential is clear for a long future on their current routes with some thought and good engineering.