I know that's why they're doing it, but the "no fans outside the stadium" thing seems to be a happy side effect, particularly for fans of clubs like PSG which care infinitely more about the Champions League than their domestic competitions. If you're willing to stipulate that UEFA and the big clubs are always going to be money whores and not reduce each round to single-legged ties unless they absolutely have to, the current solution seems pretty OK: everyone who flies to the neutral country will be getting tested multiple times and not mix with many if any locals, and the risk of super-spreader events outside the stadiums is significantly lessened.