When I install TWRP now and have no way to install a newer fastboot version later
Did you try or what is the ground for that your statement?!
You can eg downgrade from the newer Recovery ROM to an older Fastboot ROM. Download and unzip the Fastboot ROM, and flash via Fastboot with the bat script (sh script on Linux), and do not use the script for update but the other one that will Factory reset
The fact that you have TWRP installed, does not prevent or block Fastboot in any way to do the flashing. And Fastboot ROM script will overwrite your custom recovery
And the same will apply when/if Xiaomi.eu Fastboot ROMs will apply for MIUI 14/A13
Hence, where is the problem (if not just not understanding how things work)?
PS: Same if you want to go back from Xiaomi.eu to the stock.
Download that 'full' Fastboot tgz stock ROM and flash by MIFlash.
But equivalently, un-gzip and un-tar that tgz firmware and inside you will find bat and sh scripts for manual flashing through Fastboot just like in the Xiaomi.eu Fastboot ROMs
If your statement would be true, nobody will be able to go back from Xiaomi.eu to stock (even to the latest stock with MIUI 13)