CS is the perfect example of what youd have to put in place if you wanted compulsory reservations with controlled access to the platforms and therefore the train to ticket holders only .
Youd have to remodel so many stations and/or have long distance only platforms
I'm not advocating it, but you'd not have to do anything of the sort. You'd simply introduce a penalty for not having a reservation, e.g. a base £50 Penalty Fare. Some may of course consider that being a worthwhile price to board the train without one if it's urgent. Or taken to extremes you'd stop selling tickets without reservation entirely for the relevant routes, i.e. only sell Advances, so anyone on the wrong train would not have a valid ticket.
I know this Forum loves introducing objections to perfectly viable ideas, but plenty of countries the world over, including most notably the "Romance" style high speed rail operations, have this concept in place, including open stations with no gatelines at all, and it works just fine in those countries. It may not be a good option for the UK, but it
is feasible, and demonstrably so, so no argument suggesting it isn't is valid. The arguments against it that are valid are purely those of the inconvenience vs. the guaranteed seat.