Guess i am of a diffrent oppinion shoe, i think finisher will make them far harder hitting than flank. Its less situational in my oppinion.
Finisher vs Flank is variable, depends on whether you can reliably cause ranged damage (not a given in a Steelhead theme) or only want to send in cavalry after a jack has done melee damage.
But I can't see the cavalry losing Flank. It's part of the 'whole greater than the sum of its parts' stress on interactions between Steelheads. This is the one theme for mercs where PP can set up something like a professional military and I can see them stressing that, rather than weakening it.
Not sure about Point Blank over Assault. Point Blank will be available outside of charges, which might be too many attacks (PP does seem quite happy to make most cavalry considerably stronger on the charge). I also quite liked Assault in Mk2 when the cavalry were good, as it gave a ranged threat extension which was sometimes worth it).
Point Blank attacks also wouldn't benefit from Paymaster - not a big issue, but might be something PP want to avoid as a complication.