Forget the rational arguments, it's election time!
There's not all that much use taking all pre election promises seriously, as most are for the purpose of getting votes and will never materialise. If this wasn't the case then we would already have the bestest transport, healthcare, education, etc in the universe as that's what every party has promised before every single election I can remember.
On this basis alone, you have to admit it's quite a stunningly good headline grabber, and it's been amusing watching the response to it which can be fairly characterised as "It is outrageous and irresponsible to promise free things like this which would cost a fortune, unlike all the brilliant slightly different free things which we are promising which won't cost anything". Reading between the lines of some of the reactions by politicians, I also sense a slight bit of "damn, wish we'd thought of that one....".
Personally, I agree with a few other posters so far who have made the case that Openreach should really be taken back into public ownership, as it is effectively a poorly run state mandated monopoly anyway, little more than a compulsory rent seeker.