If the hardware is the same, why cannot a v4 be upgraded to v6 without the unprecidented step of a tradein program?

Iirc that backplane is similar, but v6 works to much higher specification which changes the way that cards needs to be terminated. Some v4 cards are compatible with this, some are not.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).