Giving existing users access to WiFi 6 would not only mean handing out SH3s but also mean rolling out new WiFi 6 WH/Complete WiFi discs for those that use them. For obvious cost reasons I can't see that being a high priority for BT/EE for existing users without upgrade charges involved.
But then, how relevant is WiFi 6 to the masses right now? The concensus on the interweb is that WiFi 6 isn't really necessary except, maybe, for those with 900MB+ BB connections.
I considered this and ended up taking the plunge on a 3rd party WiFi 6e mesh, keeping the SH2 as a FTTP modem only. The 3rd party router doesn't need it but I kept it only to allow Digital Voice through my Panasonic phone system and Hybrid Connect.
The 3rd party router has a fixed IP address in the SH2, to allow me use its built-in OpenVPN server it is in the SH2's DMZ so I'm not even using the SH2's firewall and it is now the DHCP server for my WiFi mesh. I only keep the SH2 WiFi as a sort of 'Guest' network as a courtesy to occasional visitors.