If the Welsh Goverment want TfW to make full use of the wires (to save fuel and make the business case for electrification stronger) they'll be wanting bi-modes on the Swansea-Manchester route. Given it is planned to be five coaches that is two units per diagram, at least 18 units (I think Swansea-Manchester would require 9 or 10 diagrams) if the additional mark 4s hadn't been brought in. Not a good look for one of TfW's flagship routes (SWA-MAN) to be burning diesel under the wires for over 40% of the journey wouldn't you think?
I know they go through to Carmarthen/Pembs at the moment, but I've always thought of it as two seperate routes joined for operational convience only (and because there isn't the money / rolling stock to properly provide an hourly through rail service between Carmarthen and Cardiff).
Medium-long term, yes there should be bi-modes.
At the moment, not every Manchester-West Wales service is being split at Cardiff, only 1tp2h are (IE: the loco-hauled ones). It would not be popular to split everything at Cardiff, and neither would it be desirable from a platforming perspective.
Say one alters the pattern to be 1tph Manchester to Swansea plus 1tph Cardiff to West Wales (Milford/Pembroke/Fishguard).....that would give three fast trains per hour between Cardiff and Swansea, plus one Swanline. That's probably overkill. Plus Grand Union plus freight......ehhhhh.
So, trying to keep the 2tph fast between London and Swansea, but keep the link from West Wales to Cardiff......1tph Manchester to Carmarthen? But then we've already got 1tp2h GWR turning around there, plus Grand Union (potentially). Adding another service turning around Carmarthen would probably mean there are no platforms (it's only got two). So extend the Manchesters all the way to Milford/Fishguard/Pembroke? Would it be a reasonable use of 5 coaches to do that? TfW aren't going to be running all 5 coaches to Milford Haven when the 197s come in. Does the infrastructure allow it?
TLDR - Working out where to split that service that provides a good solution operationally and for passengers is difficult. There are solutions. No doubt there will be investigations when the scheme becomes more concrete as to what the service pattern should be. You'll probably see other routes going from 197 to bi-modes/electric first. But.....who knows? I'm still assuming the service specification will be essentially the same as it is for Dec '24. Come back to me in 6 years.