OT should work just like siege in warband... so long as you're close enough to the flag to cap it, OT continues. As soon as you leave the vicinity of the flag, OT ends immediately. There's no reason why you should be able to hop on the command point, reset the OT timer, and hop off again and again. OT should end as soon as you step outside the boundary of the control zone, or die of course.
The way it works with M&B has that weak point that you noted: the draw ends immediately as soon as you leave the vicinity of the flag. That would be extremely brutal, and break the flow of the struggle, if it simply ended the round. At least with the timer you know that none of your teammates is close enough to the objective, so you need to go and hug it, there is a reaction-action gap available; if Overwatch worked like you said, you would have all the heroes hugging the objective because they could lose the round as soon as those who does are killed (immediate ends of OT) and with the current size of the OT area, a good half of the heroes would be near useless and it would be a completely chaotic brawl. With the timer they can hop on and off skillfully, but thats pretty much all they can do, but that liiiiiiiiiiiiiiiiiitle window of opportunity permits all heroes to try and fight for the objective, be that those who want the OT to end, or those that want the OT to continue.
The timer could be decreased by 1 or 2 seconds, or another delay mechanism used in its place, but immediate lose would be so against the flow.
