and are probably already using non-approved DIY approaches based on GPS anyway, as it's perfectly legal...until it's banned, anyway
I don't think that a whole lot of pilots are doing DIY letdowns, using GPS alone, to the sort of levels possible with ILS i.e. 200-300ft DH.
And if you do a letdown to 1000ft AGL (which is only slightly above some NDB approaches) you are still legally at the MSA. Even the FAA allows IFR flight AT the MOCA.
It's the area between the two levels. Is there any statistically valid evidence of problems there?
The bottom line is that it can't be enforced, because nobody can tell when the pilot got visual. Except possibly at a towered field which has a cloudbase measuring device, and even then only in pretty extreme cases.