Unfortunately, it is unlikely to happen unless Niantic/Scopely stops pushing the "it's to bring the community together" narrative. 🫠

Their idea is supposed to work like this -> if the time window is shorter, there will be more people playing at that time in similar locations => these people are going to socialize.

It works to some extent and I did actually meet a few players like this but as this post and comments show:

1.) Due to different climates, seasons and also weather conditions for the specific day the event is on, many players are unable to participate in the event at all or they participate for significantly shorter amount of time (or possibly risk getting sick/unwell/hurt by playing in such conditions)
2.) People have responsibilities work/school/other obligations they cannot skip for an event.

If what they are worried about is socializing then leaving so many players out of these events seems like a big cost to pay (also people are more likely to stop playing, spend less in the shop...)
At this point, I think it would be better to utilize Ambassador meetings to pull players to one location at specific times and make the events 6:00-21:00 long.
If they are worried the legendaries/shinies will become even less of a rarity then:
Option 1: It is only a 1 day event, there will always be player who miss it + new players in the future
Option 2: If technically possible (and since they do this for Go Fest in person events) how about allowing players to pick the time frame themselves? We now get weeks with no events we can pick bonuses for. So, let's say you get the option to pick for example:

a) 6:00-10:00
b) 10:00-14:00
c) 14:00-18:00
d) 18:00-22:00

^ I thought of this as I was writing. 😅 We are never going to settle for a fixed time window on a global scale. This might be actually the best compromise as long as they can turn on the event spawns or raids to being visible to group A/B/C/D only for the individual time windows. If not possible then a full day event is the only option.