If models didn't sell out, there would be less incentive to do more runs, ironically, so the selling out of a model is a good thing, not the opposite. While that does sometimes mean the price of second hand can rise over time, particularly in the case of desirable, or rare or limited run models, such is the situation in all fields of collection / modelling / hobbies. Even a 'small' run of 504 locomotives, assuming all the tooling costs have already be absorbed (am sure some have not yet been) will cost north of 75,000 Euro by the time they land (if not more these days with the costs in china increasing).
These days there are few that will spend that kind of cash on a 'speculative' re-run? As has been highlighted, the market is small, probably closer to 600-700 'buyers' of Irish stock these days, and even that small number likely represents a doubling in the last 5 years, primarily thanks to the work put in and excellent models produced by paddy Murphy, Leslie (Provincial Models), SSM, Glenderg, DRM Models, Irish Freight Models and many more! A run of 504 of any one model is 'about right' I would think for this market.