It's not quite so simple Max.
35b increases speeds by opening up more bandwidth at higher frequencies, but it also relies heavily on vectoring.
It also works best on newly constructed sites ie those without any other DSL services. To quote Huawei
In a reconstructed site, the original services and users interfere with new SuperVector services and users and the interference cannot be eliminated.
Ideally all the users and services should be brand new. Although it is possible to reconstruct a site for 35b all users lines must use SuperVectoring. What are you going to do with the LLU adsl2+ lines. The likes of Sky & TT would have a hissy fit and OFCOM would likely pander to them.
You are also forgetting that whilst the Huawei MA5616 are classed as being upgradable for [normal] vectoring, you practically have to rebuild the DSLAM with new & replacements parts. As I've previously said in
another thread, it would be easier (& cheaper) to upgrade the ECI M41s to V41s than it is to rebuild a Huawei MA5616 in the field.