If the 4K pre-order had been very limited - like the 4K THE GREAT WALL (500 reportedly) - then that one would be showing "sold out" first (like THE GREAT WALL).
As 3D discs have been around in the U.K. for a good while (since mid 2010) and true 4K UHD discs only since the third quarter of 2016 stands to reason that more people have 3D capable TVs than 4K TVs.
Also stands to reason that 3D discs will be produced for a good few years more . . . even if all the manufacturers abandon 3D TVs . . . and a high quality TV, as in expensive, manufactured during the past few years, with 3D capabilities, should have a useful working life of at least 10 years (for example, my TV manufactured in 2006 is still going strong although I sadly had to abandon my 2008 Blu-ray player a year or so ago and buy a more modern and far less attractive one).
1080p Blu-rays will NEVER be replaced by 4K UHD discs but I welcome the new format running with 1080p as I'm not a Luddite and any improvement in PQ / AQ is something that should be strived for but of course the vast majority of films that have made the leap from DVD to Blu-ray will never have a better presentation than 1080p.