future increases will largely depend upon basic economics.
Fares keep going up, but so do passenger numbers (apart from 2017-18. but apparently that was a blip and they are now rising again).
If passenger numbers started to fall continuously, then I assume the government would re-think the policy. But despite all the media criticism every single year (actually, 3 times a year. Once when the inflation rate is released for the appropriate month, then again when the fare increases are announced, then again when the increase actually happens!), passenger numbers are still strong.
The policy is of course that passengers are expected to pay an increased share every year. But even in a theoretical situation, if fares got so high that they funded the entire cost of the railways (how many years would this take?!?!), fares could still keep rising, with the extra money being paid back to the government in premiums. Give it a thousand years, and maybe the railways will eventually pay back their entire cost for past, present and future!