E300's will be gone with E320's not that far behind with somthing newer replacing both, possibly same model as HS2 trains
E300s will indeed be gone, but I think some elderly e320s will be going strong, given the e300 entered service in 1994 (30 years ago). If the same applies to the e320, they will be in service until at least 2045!
An AT400 replacement for e300/e320s is an idea, but it would be more exotic to see the TGV M or another future double decker TGV being used as a replacement.
With HS2, the planned AT400 trains will indeed be in service in 2044, and I think there will be a second generation of hs2 trains I'm service, which could be either single or double decker. I would also want to see the scrapped HS1-HS2 link be reinstated.
On the subject of 484s, a good thought experiment would be imagining which other rolling stock could be upgraded/rebuilt/converted by 2044, like how the class 56 was rebuilt into the class 69?