As I am sure you recognise the barrier to a true scale 4mm is not technical. It's about space requirement. That's why we have HO mechanisms in OO models....True track gauge in 120 scale is 11.9583333mm, an error of 0.04 of a mm or 0.35%. So it well and truly overcomes the "better track" argument. It is better than both OO and O gauge in that respect.
If they can get this close to the correct track gauge in TT then it demonstrates that there is absolutely no excuse why they cannot mass produce P4 !...
The reason we have HO mechanisms in OO model is because the thicknesses of plastics and size/strength of valvegear is such that it is difficult to make them strong enough if the mechanism is correct gauge (ie P4).As I am sure you recognise the barrier to a true scale 4mm is not technical. It's about space requirement. That's why we have HO mechanisms in OO models.
Personally, I think the 'critical date' was some10-15 years earlier. If Lima had made its HO scale models in the 1970's to the same 'standard' that they achieved in the mid-late 1980's when we only had Lima and Hornby wasn't doing much, I think the whole proposition of British HO could be very different today. This would have meant that when the fixed link to HO-land opened, the UK could already have been HO and British products today would be running with compatibility with their European counterparts.If some brand had been brave back in 1990 with the fixed link to HO-land opening, and had launched contemporary D+E models in HO, by now we would have no scale/gauge problem in UK RTR modelling, fior the period since steam was withdrawn. UK HO D+E will work just as well as all the rest of the world's HO D+E.
It will be interesting to see what happens in 1:120 TT. They are starting with correct track. One wonders what compromises will be made to accomodate the narrower UK loading gauge. But at least they have the track gauge right! So long as we don't end up with 3mm scale track and 3.25mm scale bodies!!!The recidivists like myself wanting steam, would still be in OO if using RTR. All the UK's narrow width dimension, thus less space for outside valve gear, and close fitting splashers on driving wheels limitations, which forced the OO compromise will equally apply to 1:120 TT.
Resetting the minimum radius expectation is long overdue. We are either building a model railway or we are building a toy trainset. If you want to build a trainset, then I'm afraid that trainset curves require compromises in the models. If you don't want models compromised, then don't use toy-trainset radius curves!This makes the choices for anyone venturing steam models in TT 'interesting'. Might it be the case that those wanting Walschaerts gear pacifics and similar size locos are told 600mm minimum radius, or some similar figure? That would enable correct exterior appearance, no need for the 'vari-scale' approach of stretching the width dimension. With a new introduction, it should be possible to 'reset' minimum radius expectations. (Your inside cylinder 0-6-0 will be quite happy on 200mm radius or thereabouts, so the choice lies with the layout builder - compact curves = small locos - if you want a main line with big engines then much larger radii are necessary.)