I think its because if you are going to rewheel everything, you might as well get the gauge right , Why would 5'3" end up at 20.2 by the way, the check gauge ( i.e. flange gauge ) is not a function of track gauge per se, i.e. you can have 21mm with 1mm flangways ( Templot will do this quite nicely )
and in my experience nothing is " drop in " going to 21mm EM or P4, often the backs of W irons have to be modified, bogies modded, etc , longer pinpoints ordered ( which are getting harder to source ) , rewheeling bachman, split chassis stills requires skills etc . its all a reasonable effort and if you own a lot of 00 stock , its a big undertaking
My own view on 00 is that the height of the track contributes to the incorrect visual effect , then you add the flat bottom track effect , where the rail is sitting on the ballast, .Code 75 bullhead is now available from four supplies including PECO, and that when ballasted looks far better then what was available in the past, the bullhead rail, sitting above the ballast also gives a " lightness" to the track.
I have tried C&L, SMP and the new DCC Concepts code 75 bullhead, the new PECO look a real winner and Ive some on order. Points of course remain a problem.
This brings us back to Noels pic of Adavoyle, in my view low profile 00 bullhead, properly ballasted and blended in , can actually look very convincing
dave