All when and good but when folk (fans, broadcasters, whoever) are spending £000's having more than 1-2 days to plan and move is probably pretty beneficial.
Ah, never thought of it from that point of view.
Play the last group games over 2 days (8 per day at 4 time slots 1, 3, 6 & 9 or something) then there's a 3 day break before the last 16 and time for people to sort travel
You know where the games will be played, you'd just not know which teams will play - which you don't really know now either, as it could be any of 16 outcomes from two groups going into the round of 16, for instance.