I feel like Rome>Normans and various other pathways are easier to accept if you don't think of them in terms of the people, but rather in terms of the geographical location and of the infrastructure.
Civ has never really been about the people; they tried it a bit in IV, but otherwise it has never been concerned with ethnicity, migration, genealogy, etc. Civ has always been very simply about the placement of settlements, farms, roads, etc. on a world map. It's about building a footprint of your civilization in the fictional geographic location that you are given. All very macro, all very abstract.
If you think about switching & historical pathways in terms of the people, then I don't think it will ever make sense. But if you think about it in terms of a place in the world, a place in which different peoples come and go, where these different peoples build things, sometimes new things, sometimes on top of older things, where these people impact and shape the landscape around them, then I think it does make some sense.
If we take the Britain example again, last time I read anything about it, I think it was fairly well understood that the Romans had little to no impact on the gene pool of Britain, but they did have a lasting impact on the landscape, e.g. establishing Londinium.