If una cuм masses are mortally sinful and you just so happen to be the only non-una cuм chapel within a reasonable driving distance, then you have a monopoly on the faithful's checkbooks
May be true in certain locales, but there are also many Traditionals out of reasonable driving distance of any traditional Mass whatsoever. As n.o. dioceses are first eliminating Latin Masses and combining dioceses into one, the same with parishes and those remaining not having Masses every Sunday due to lack of priests, Catholics of all sorts increasingly find themselves in the same predicament.
Your statement, also, is predicated upon most traditional Catholics having an una cuм traditional Mass, also, upon being able to drive what you call reasonable distances and upon having a car or the ability to move. Some have cars, but the expense of travel is too much. Others cannot physically undertake hours in a vehicle. Chances are, if you can’t afford to travel, affording to move is unlikely.
These and similar factors are why my personal policy on finer points of belief that cannot be settled without a valid, Catholic, and good Pope, is “Don’t Ask; Don’t Tell.” If someone with authority, ie, the priest, questions me directly about my stance and cannot accept it, therefore does not accept me as being Catholic, I leave. If Susan or Chad from the committee or Mrs. Dee Mentia interrogates me, that means nothing. It’s different if there’s a priest who insists I confess to something as a mortal sin, when it is/was not, any sin at all, then I leave. I’m not interested in cults be they of false doctrine or personality. I do not hear Mass where I am obviously not wanted, why upset their apple cart?