For some comparison, This year at TCS nationals 17.5 gt2 class the fastest lap for a modified slower layout of Tamiya America is 13.8 lap. During practice, top 5 guys in gt2 can do around 13.5 to 13.7 second lap on normal track layout. Mind you this is with the poor B3 tires. This is less than 0.5 second slower than a competitive 10T mod cars in the brushed era. 13.5 blinky GT1 cars can hit high 12 seconds on that track which is matching the old mod 10T motor cars can do on Sorex tires. So yes the cars definitely got faster over the years with brushless power.
But since USGT and VTA are already in place, why not suggest these classes for beginners? By the way the competitiveness of any class is only determined by the skill of drivers in that class. You certainly can have a bunch of sponsored driver in VTA and making it impossible for new drivers to make A main in that class. I feel it is a good rc etiquette for people with better skill to move up a speed level so the class can be fun for the less experienced.
As for some reason people stay in 17.5 in America I think also has to do with part cost. It is harder to break a car in 17.5 than in mod if you make a mistake. And parts wear much faster in faster classes. And most competitive drivers here drive off road.
And please do not try to create more classes, it just dilutes the hobby.