Much reduced passenger usage beyond Southampton, and withdrawal would make one less unit needed for the service? So they probably just found a better use for the unit somewhere else.
Now, if an extended Victoria was reintroduced (with the current SN timings), it would be following within 10 mins behind the xx51 from Southampton, which itself follows the xx43 XC, so 3 of the 4 fast tph Southampton to Bournemouth would be pretty much packed together. From Brockenhurst to Bournemouth a SN service would then either be right behind or right in front of the Poole stopper depending on how they pathed it.
If OTOH you extended the Brighton beyond Southampton, it would be running just ahead of the SWT xx24, and xx30 to Brock.
No easy answer.