The 373's have a key weaknesses that are probably leading to their demise: -
1 - They are built to the cramped UK loading gauge rather than the larger European loading gauge.
2 - Their unique electrical system built to allow them to operate on the Southern DC 3rd rail and old BR signals, added to the SNCF and SNCB systems make them complex to maintain and restricts their usage.
3 - They are a small, non-standard sub set of a much larger TGV product.
4 - Unlike the class 374 or Thalys TGV sets, they cannot operate into Germany or the Netherlands, thwarting growth on Eurostar until the class 374s are ready.
They may only be just over 20 years old, but they no longer provide what any train operator needs, a flexible, economical train to operate, that passenger like. HS1 / CTRL was their death sentence, along with problems with snow and being a small unique bespoke fleet for which the need went 10 years ago!