In my opinion we are now in the 'zone of opportunity' for models of pre-group traction, because most of the significant grouping and later loco designs are available. If that's accepted, then the next question is how long the demand holds up against the inevitably steadily rising prices (a move away from dependence on China may ameliorate the latter, Roco are trying Vietnam). I am optimistic about the demand side for at least another decade: there are still plenty of folks yet to start drawing final salary pensions who were young when model railway was a major hobby and might just think of model railway, round 2. So all being well some of these retired folk looking for an interest that can be pursued when the weather outside is frightful, will maintain the grey pound spend on model railway purchases out beyond 2030.
Then there's hope that the two 'current standard' models of Scottish 0-6-0 locos (alright, the CR specimen isn't here just yet!) will push that particular door open, and the typically good looking products of St Rollox and Cowlairs will get more attention from manufacturers. I'd love a D34 even though it doesn't belong in the KX area, to run alongside the equally inappropriate J36 Hornby supplied, and the similar charms of a CR 4-4-0 would likely appeal to many. And both companies have attractive and long lived tank engines to offer, mentioned up thread. We must BELIEVE...