The fact that Heljan got as far as they did with the 31 indicates "8 years" might be overly pessimistic, I suspect the other manufacturers are taking stock of a) Hornby's development pipeline timescales first and b) the uptake of the scale. Duplication this early in the game is exceptionally counterproductive.
Heljan were unfortunate to have been pre-empted with the 31, but they are probably well set up to get one or more of a 20, 25, 26, 27, 33 or maybe even a 24 to market within a 24 month timescale if they had the confidence to do so.