What you have to do (just like with flights) is to book the latest one you are likely to need, then move it earlier if possible. If not possible you go to the pub. It isn't that much of a problem.
But again, why would you introduce such a system?
A couple of days ago me and a friend decided to make a short notice, short (ish) distance trip on a long distance service. We could turn up at the station, knowing exactly how much we were going to pay, and that we could get on the train we wanted. We stood for one stop, were lots of people get off, so we sat down from there. We then stayed at our destination and decided to do something unplanned. Having flexible tickets though, it didn't matter we just got the train an hour later. Again at the busy station there were a few people standing for one stop, where we got off.
I assume the compulsory reservation advocates propose one of two options for this service:
1. Completely reservation only. We would have to go to the station, not knowing how much we were going to pay, or if we would be able to get on the train at all, and whether we could get a sensible train time coming back, as it's a busy line where lots of bucket and spade traffic go the whole length, coming back in the evening. When we decided to spend a bit longer, we would have had to lose our money on the return portion, and hope we could get reservations all the way to our final destination.
2. Partial Compulsory reservation: The train we were on would have maybe 1 or 2 coaches for people without reservations. These would be crammed full.of standing passengers and very inconvenient. Also, if the train was full at the start, people would just split tickets as they do now, so they can travel in the unreserved bit for the journey they're making, rather destroying the point.
Both of these options are much worse than the status quo, which is a much more pragmatic option. Also with either of these options, we would've just drove and not bothered with the train at all.