Cars like that Bently and other execuitve saloons are very heavy these days so the extra weight and the power mean its unlikely you will get more than about 15K out of a set of tyres taking everything into consideration.
I don't know what the legalities are in the USA, but in the UK, the legal tread depth requirement is 1.6mm and new tyres come with roughly 7-8mm of tread. There are so many drivers who fall far below the 1.6mm limit due to things such as bad maintenance, worn suspension, incorrect whee alignment etc its shocking as I have noticed that a tyres performance starts to deteriorate below 2mm in general.
I have never had more than 15K out of a set of UHP (Ultra High Performance) tyres (like Goodyear F1, Conti Sport Contact, Pirelli P-Zero Assimetrico etc). I do drive enthusiastically, albeit not to the limits, but I do avoid flooring my car in 1st and 2nd gear and generally I get around 12K out of a set of tyres on the driving wheels which in my opinion, is quite accepatable.
Who can say, that driving normally, as you do everyday, that they get more than 20K out of a set of tyres fitted to the driving wheels before they fall below the legal tread wear limit?