Although I am a big fan of lockable ports, I am not sure whether this fits well to a mobile device like the BBF, which has been designed for mobility.

Maybe RME's intention (not to use lockable XLR ports) is here to offer a mobile device without any sharp edges,
which you can put into any bag without damaging something (clothes, etc).

Regarding your point changes every 4-5 years, this is most likely pure occasion, as RME devices have usually a longer life cycle.
Many things can be changed in the FPGA by flash upgrades and also driver upgrade.
There were surely other drivers for upgrading BBF and BBF Pro than simply to change the product regularly every 4–5 years.

In my opinion, the BBF Pro was a major overhaul of the BBF after its successful launch as a mobile device and brought many useful changes. Then RME surprised with another update, again with very useful changes, among them faster converters, FS clock, two switchable reference levels for Main Out and many other details.

I think now the product is near perfect so that there is not much left to be optimized further.
Keep in mind, that USB2 bus power also results in certain limitations.

I know why you mention this, because of power, but I think USB3 is here not in scope anyway for some other reasons.
USB3 would not only bring benefits to the customers:

1. many people will be happy that they only need a USB2 port, in cases, where only a few USB3 ports are available and already in use for other things. For example on laptops or older systems.

2. USB2 has usually no compatibility issues, nearly all USB2 controller seem to be compliant to USB standards to a high degree. If you look into RME manuals, then there are only USB controller incompatibilites mentioned for USB3. As if the implementation of USB3 is more complex and some 3rd party chips are not compliant enough to the standard.

3. USB3 allows for 5 m cable length, USB3 cables allow only 3 m and have often double and tripe shielding which can make the cable a little more stiff.

USB3 better only in cases, where it is really justified, where you require it for bandwidth reasons (number of channels).

Finally, saves some production costs, also cables are less expensive. No major reasons, but a nice side effect.