I agree that your through ticket should be continue to exist and that your rights would therefore be protected. Only the cross-London element would be missing (use contactless or oyster as you prefer), and the through ticket would be cheaper by at least the same amount (£2.40 for an adult). There would be no need to have ‘split’ rail tickets.
And when we’re past the initial transition period, what then? The logic of the decision to preserve rights will be forgotten, and there will be a whole new cottage industry of avoiding honouring contracts as admin teams struggle with a further layer of complexity. A through ticket means a through ticket, with one ticket covering an entire journey. Rebooking, even with Oyster or Contactless, is not a through ticket.
That, by the way, is for the adult full fare payer. As others have pointed out, the implications for railcard holders, group discounts, etc. are wider - not to mention the simple confusion caused to irregular travellers as they find their “through ticket” does not work the barrier and have to pay “extra” to do the journey they think they’ve paid for.
This is an awful idea, driven by dogma and disregarding entirely the needs of the customer.