Firstly, you seem to be assuming that changes in Standards/regulations are always in the same direction - that of 'tightening', whereas, certainly in the case of BS7671, there have been 'relaxations'. Hence, in relation to some matters, the superseded standard is not "inadequate" but, rather "more than adequate"!PBC, you seem to be missing the point about supersession. The fact that a standard has been superseded means that a group of experts has decided it is no longer adequate. By implication, a new installation to the superseded standard is also inadequate.
Secondly, and I think that this is the main point PBC is making, in order to demonstrate compliance with Part P, one 'merely' has to present a reasoned argument (based on whatever) that one has made "reasonable provision for safety" etc., regardless of the views of any one particular group of experts at any particular point in time. If the legislators had wanted to make compliance with (the current edition of) BS7671 mandatory, they could have done - but (perhaps at least partially for reasons we have discussed) they chose not to do that.
Kind Regards, John