Is that fair? I don't know and neither do I see it as even important. It is certainly better than the previous arrangement where almost anybody could and did issue owners with evidence of dating and/or provenance. The net result is, at long last, a tightening of the rules including inspection of a vehicle when applying for an age related number or original registration, which was well overdue. I never understand why people try and figure out how to work their way around these kinds of regulations. Just look up the regulations, speak to DVLA and follow their rules it is actually not complex.
Remember that in many cases vehicles which require an age related number, or which are repatriated from foreign climes have no record of registration and/or taxation, which I believe is the reason why DVLA have decided to use the manufacturing date. This of course works in favour of the owner in just about every case.
31st December or 1st January? It is no different to any fixed point in time. Try using your debit card the day after the expiry date...........nobody seems to have a problem with dealing with that point in time.