The prices of individual tickets in the split can be raised or routing options restricted to make them undesirable or impossible.
Taking XC as an example, it's not really possible for them to raise the price of the shorter distance fares that can be strung together to make medium and long distance journeys cheaper.
For the fares they do control, they have largely already increased these as much as they can - for example by removing the Off-Peak Day Return for flows like Stafford and Stoke to Manchester or Leamington to Coventry - leaving only regulated fares, and fares more expensive than regulated ones (e.g. SORs).
I'd say most of the 'juice' out of such changes has already been squeezed.
Or open calls can become restricted calls, which can be extremely aggravating. A textbook example of making a product or service worse in order to extract more money from existing customers.
There are a few cases where this has already been done, e.g. Avanti have some ridiculous set down/pick up only restrictions at Rugby, Nuneaton and Stafford on their Manchester services. They seem targeted towards the trains most likely to be used by business travellers.
They nominally prevent contra-peak journeys (e.g. getting on at Nuneaton on a train leaving Euston in the evening peak) but in my experience the trains are still shown at the station departure boards - so it's blatantly obvious it's just a wheeze to try and stop people being offered split tickets online.
Similar restrictions couldn't be easily introduced on most CrossCountry services since they serve many local flows, and they wouldn't want to lose out on their share of the revenue for such journeys. Whereas Avanti are going to get the lion's share of Rugby to Manchester revenue anyway, by virtue of the fact they still have lots of scheduled direct trains at other times of day.
Not wanting to derail the purpose of this thread but it makes me think: "how did UK raulway ticketing get this messy and this complex"?
In the 1980s, I travelled on Saver, Supersaver and Rail Rover.
In the 1990s, I commuted with an annual Gold Travelcard.
When and how and why did it all get so convoluted, so as to create an entire sub industry of the split-ticketing concept?
Confused of aged latter-50s.
There are lots of things the privatised TOCs have done to raise revenue, which has made splitting more worthwhile. Dramatically increasing unregulated fares such as Anytime Returns, abolishing cheaper fares such as Day Returns, and making time restrictions more onerous for example.
Some things that are positive for passengers have also contributed, for example introducing new Railcards (e.g. Two Together) which come with their own time restrictions, as well as rolling out Advances (Apex under BR) to cover a much larger proportion of journeys including many local ones.
Ultimately, split ticketing exists in many countries and also existed under BR. It's less common/necessary in countries that do single leg pricing (i.e. most places other than Britain) but whenever there's a time restriction, e.g. "not before 09:30", it's likely that splitting will reduce the cost for medium and long distance journeys. That's the case in Germany, Belgium, the Netherlands, Austria and many other countries.
All of the products you mentioned above still exist in a similar form, it's just that there are now more alternatives to choose from. That choice is part of what makes splitting worthwhile (e.g. there may be a cheap Advance for one part of a journey but not end-to-end).
