They actually used to have a Qualy format along those lines, although they tinkered with it a bit. At first it was aggregate, and as one session had to be done on race fuel (you could refuel in those days) it meant the top teams tended to sacrifice grid position for a better fuel strategy. It did now and again set up some jumbled grids (especially if it rained during the session). See the 2005 Japanese Grand Prix, it actually made for one of the races of this century. Now and again a lower team would put in a banzai lap with a minimal fuel load just to get up front.
My problem with reverse grid races are that it works fine and dandy ina spec series like F2, even in BTCC where aero makes very little difference. But, these cars aren't spec. Sure, you'd get different race winners. At first, anyway, until the engineers got clever. But it'd feel entirely artificial. It certainly wouldn't feel as special for a driver like Gasly. Plus, it'll mean extra strain on engines, probably resulting in more money having to be spent producing extra engines. At a time when costs are at record highs and the sport is looking to cut and cap costs. I just don't see it as a completely workable scenario.
