Might be popular with city fund managers (I wonder why) but any regressive change to the ISA regime would be very unpopular with the public.

Savvy passive investors pick a global index fund for max sustainable rtns, where US market is dominant anyway, eg 65% VAFTGAG
https://www.cityam.com/city-minister-takes-aim-at-cash-isas-in-call-to-back-the-stock-market/