The XJ has sold only 1,400 Units,and The Audi A8 has only sold 1,200 Units as of April 2015. Still nothing to cheer about,since the Audi is down 10% this year,and the Jaguar is down 22% in sales this year,with the Luxury market being red-hot.
Flagship Vehicles aren't expected to have the volume of sales as an entry-level, or mainstream model,the XTS isn't a Flagship Vehicle,and the comparison to the S-Class/7-series/XJ at their prices,is unfair to the XTS. Those models SHOULD BE more refined,and have superior horsepower,considering their difference in price!
Yet,you put cars like the Equus into the Luxury Flagship conversation,and it has no pedigree,little prestige, or history in selling premium vehicles to the American market. The XTS is priced more like the E-Class and 5-Series,and sized more like the Short -Wheelbase Mercedes and BMW models. Either you're getting much more car for the money,with the XTS,or you're being drastically overcharged for the same size vehicle,in the body of Jaguar,Mercedes,BMW,Lexus,etc...
If the so-called prestige and exclusivity is real enough for the Customer to perceive that the extra money is worth it,more power to you,but you might want to look at the depreciation values of BMW/Benz/Jaguar/Lexus;the 3-Series,5-Series,Audi A4,are all in the Top Ten for most depreciation! The S-Class sales are down 2.9% this year,the E-Class sales are down 33%,A8 down 12%,XJ sales are down 24%,LS Lexus sales are down 14.5%,and Panamera sales are down by 25%! the XTS has an increase of 13.6% this year. Audi turned it around,just give GM some time.
The Top 10% of the population has helped set the Luxury market on fire,but the demographic for the Luxury Car category is not the same as the Large Car category. Most people won't cross-shop the XTS and MKS against the S-Class or 7-Series,but it's done here all the time.