So back where you started, all 230 volt electrical equipment is potentially dangerous so no one can claim that anyone who gives a C2 is wrong.
I don't think that even a Court would be that silly, given context, the
full definition of C2 and just a tiny bit of common sense.
The full definition of C2 is "Potentially dangerous -
urgent remedial action required", which quite clearly means something like "Potentially
more dangerous than it should be, hence urgent remedial action required". Even you surely do not believe that, 'by definition' all 230 volt electrical equipment "requires urgent remedial action", or that all motor vehicles, all tools and, indeed most household objects "require urgent remedial action" - just because they have the potential, in some situations, to be 'dangerous'?
We all seem to agree that a sensible approach is anything which has ever been allowed with BS 7671:1992 when it first came out should be allowed now.
You are putting words into the mouths of some of us. What you suggest is one possible approach, which probably would be reasonable but, as I keep saying without knowing much about the requirements of BS 7671:1992, I cannot say whether I would personally agree with that 'rule of thumb' in relation to everything.
You also seem to be overlooking my comment which you said was a 'good point'. If your view is that anything which would have been compliant with BS 7671:1992 'should be allowed now' if it was installed in the 1990s, then why should the same thing not also 'be allowed now' if it were installed in 2002, 2009 or even 2020? There is no way in which 'when it was installed' can make an iota of difference to how 'safe' or 'dangerous' (as perceived in 2020) a certain practice is (in 2020) - so where is the logic in using 'when it was installed' as a criterion for determining how 'acceptably safe' a practice is?
Accepting that views and attitudes (about 'acceptable risks') change over time, we now (in 2020) either do or do not believe that something that would have been compliant with BS 7671:1992 is considered to be 'acceptably safe' in 2020. If we
do believe that, then we surely should accept it in 2020, regardless of when it was 'installed', and, equally 'surely', if
we don't believe it, we should not accept it in 2020, even if it were installed in 1992.
Any new rules set by government or IET/BSi always have a time when they come in, one for new, and two for existing things, be it a house or a car.
There are obviously impracticalities in suddenly introducing a new regulation which requires that everything (existing or new) has to comply with it 'by tomorrow', hence it is normally as you say - but the 'period of grace' doesn't have to be all that long and, in extreme cases, might occasionally effectively become 'zero'.
Even when Airworthiness Directives are issued to airlines to require them to implement some urgent safety-related modifications, they are usually given a reasonable period of time to implement that in their existing fleet but, if the danger is deemed to be sufficiently great, the authorities can (and occasionally do) 'ground the whole fleet' until such a time as the safety measures/modifications are effected.
Kind Regards, John