On that basis termating all West of England services at Basingstoke/ diverting them to Reading would be a much bigger “easy” win with EMUs instead running to London.
whilst we’re at it, bin the cross country off at Basingstoke. Would be silly to have them running over 3rd rail,
Hmmm, all getting a bit sarcastic. But it seems to me that we just accept Portsmouth-Cardiff as a 'route' because that's how it's always been. If the fast train from Portsmouth-Southampton had always turned left and proceeded fast to Bournemouth and Poole, I suspect there'd be equal derision if I suggested it should stop going to Bournemouth and be re-routed to Salisbury and Cardiff instead.
Basically Southampton and Portsmouth are big conurbations within about 20 miles of each other, which people want to commute between for work, and travel between to meet family and friends.
These two should be connected by fast and regular modern electric services, and the infrastructure's in place for that.
But the choice of train transport is either a pleasant but glacially slow EMU stopper, or the grim, cramped and filthy experience of a 30-year-old GW DMU.
And the reason for this is the 'traditional' requirement that the fast train then heads off into 100 miles of nothing-very-much (apologies Salisbury & Bath) before it gets to anywhere else of significance, taking far too long to get there to be any use for people commuting for work.
And it usually gets delayed or cancelled on the way back, making the Southampton to Portsmouth service even more unreliable.
Yes, the fast trains from Portsmouth to Southampton should be 444s or 442s, possibly continuing to Bournemouth and beyond. And the train from Cardiff should become the 'Romsey Rounder', giving any passengers for Portsmouth two chances to change (Southampton and Eastleigh).