They would still have to stand if a train got cancelled and they had to be put on another train which was already full.
Aside from cancellations, or other types of disruption which would cause problems either way, no-one has to stand under the present system.
Possibly to stop people complaining we should have an opt-out rather than opt-in to seat reservations. You get a message saying, no reservable seats left for that service, are you still happy to buy this ticket if you are not guaranteed a seat.
There have been countless occasions where I have much preferred to have to stand for part of a journey, than to have to wait and travel the next day, or be forced to buy some kind of more expensive ticket. Usually it's when I've left decisions to the last minute about travelling home at christmas, or something. It's my choice to take that risk in return for flexibility, and I don't want it taken away from me thanks.