As a very long time frequent but irregular passenger on the WCML, I thoroughly approved of Virgin's introduction of a capacity controlled fare pricing system when they took over the franchise from BR.
I had too often come unstuck, (and for example only using a more recent TPE example, but happened regularly under BR on the WCML) where when on booking your ticket you had no information on the likely passenger loading on a particular service resulting in say 25% of the passengers completing a 4 hour Manchester to Edinburgh journey having to stand for the entire journey when it turned out the next service (in the TPE example 2 hours later and both 2x185) had seats free because there was no price differentiation. You also had peculiar loading issues, for example on the successors to the former Manchester Pullman services; First Class was often overbooked whereas Standard Class on the same train was nearly empty.
What I want to see is when I am considering my journey (whether that is weeks or 30 minutes
beforehand) is what seats are available on which train and the cost of those seats (exactly like the Paris to Nice example in the post above). If the train is nearly full and all the available tickets are expensive, I will then consider my alternatives including changing journey times, even to the extent of the day of travel, and alternative routes and mode of travel (although air is no longer a viable alternative for me as it was is in BR's days, partly due to the loss of competition on the route) to primarily increase my comfort and also save money.
I remain entirely convinced that for "intercity" rail journeys a mandatory advance booking system is a priority. For the limited number of passengers who commute using the same train everyday, the convenience of having a reserved seat on your train should be a bonus. If for any reason you cannot make you pre-booked train then you can rebook for a small (and this needs to be reduced possible to zero) administration fee and release your seat for another passenger. While a small amount of overbooking can be permitted by the train operator using jump seats, notified cancellations, and a small allowance for experience determined unfilled reservations, or (as Virgin used to do) a small number of seats left unbooked for members of their "Traveller" Club of regular travellers. If you need to travel at the last minute and don't have a reservation then at your origin station you can pay an enhanced fare (the national Anytime fare) and take pot luck on being able to get a seat but accept the risk of being turned away if the train is already overbooked.
I accept that standing should be permitted in situations where there has been a disruption to service, but train operators should not be allowed to sell tickets in excess of the seating capacity of the particular train.