There are a lot of practical examples of fixed pricing or mixed fixed and market pricing mechanism for railway all over the world.
For example in Germany, R and RE are very much zone based, while ICE is market pricing
Similarly for Trenitalia in Italy, R and RV trains are distance based, while IC and Frecciarossa are market pricing
Yes, for longer distance journeys zonal or mileage based pricing is impractical hence the move to market based pricing.
True zonal pricing that exists in Germany, where there is no penalty for mixing and matching zones is sadly not really a thing in Great Britain.
Japan is a more extreme example - base fare simple distance based, train type premium (for express, high speed train) and service premium (for green car/1st class) are added onto the base fare which are also distance based.
There are railway company run OTAs that sell advance tickets at a fixed discount.
And to be fair, we do use a mixed of variable and fixed pricing in the UK - Oyster and PAYG fares in London region, return fares for some operators are fixed.
Yes fares are set by all sorts of different methodologies, many of them historical. But there has been a huge drive towards market based pricing in recent years. PAYG fares in London are loosely and nominally zonal based
but with some modes costing more than others, and premiums charged for mixing modes, which isn't how it's done in more sensible countries such as Germany, Switzerland etc.
Zonal pricing absolutely makes sense for urban areas, but zonal pricing for the entire GB rail network is a bit more questionable.
While some people seem to associate market based pricing with loopholes, anomalies and splitting opportunities, these issues can and do still occur where pricing isn't market based.
Making walk up single fare more sensible, e.g. fixed at 60% of a return fare when there is one, is certainly practical and worth discussing.
Single leg pricing, or going part way towards that, is a topic that's been discussed on other threads. For example a proposal to retain return fares but price singles at around 60% was posted here:
In the main thread here: https://www.railforums.co.uk/threads/lner-removing-return-tickets.248455/page-7#post-6218916 ...there has been discussion of the LNER single fare pricing move and how it disadvantages certain groups, as well as meaning a near across-the-board 4% increase. This being...
www.railforums.co.uk
There are numerous other threads where it's cropped up too.