Chances are in 20 years time the labour market will be very different to now with more automation and those jobs remaining being higher skilled. Taking an example from another industry people are already developing 3D "printable" buildings removing the need for construction sites to look like they do now. As such jobs which currently exist will not exist in the future and those that still exist will be different to what they are now.
With more and more automation then there is an argument that not everyone will need to continue to be employed full time to have a good standard of living. There are even some who think that there could be a possible future outcome where significant numbers of people are provided (by the state) a very basic standard of living (including things like a basic printed residence) and that people would then only have paid jobs to have luxuries (although it could lead to a lot of resentment between the different employment groups).
The first signs of automation are already visible, such as fewer and fewer young people having Saturday jobs as shops have less checkout staff.
Comparison of the labor market in the 1950's and now are very hard, as although in theory there was full employment in the 1950's there were less women in work.
Will rail staff be affected by the changes? Yes. Does it mean that customers will see a deterioration in the level of service? Quite possibly, but that would likely mean that the Rail Industry would have to change how it operates to simplify what it sells so that customers could see an improved service.
One way, is to link tickets to credit cards which removes the need to pick up the tickets from the station (or at least much shorter codes to collect the tickets when the cards are inserted, as why does there need to be a 16 digit code to collect tickets when the credit card has been presented?).
If season tickets were linked to a credit card then to renew any required paper ticket you could just present it to the contactless card reader on the TVM which then gives the user the option to make a few selections (i.e. duration, start date, etc.) and enter payment to renew. As such it could well be faster than using a ticket office window.
Likewise any recent journeys could be liked to the contactless card giving people the option to repeat past journeys by making a few selections (e.g. pick a past journey, travelling now/later, selecting the date/time if relevant, etc.) before making payment. By knowing what journeys the payment card has valid tickets for (such as season tickets or other tickets for use as part of a longer journey) it would be possible to split tickets and buy extension tickets (i.e. a ticket from station B to C when the passenger has a season ticket from A, the location of the ticket machine, to B) as it would be able to "ensure" that passengers have a valid ticket for any journey that they wish to make in the same way that staff do now (of course that doesn't stop people splitting tickets and using a train which doesn't stop at that station but neither can ticket office staff).
I do agree that there is often a need for more TVM's even before there's a reduction in available staff and that the TVM's need to be much smarter than they are at present (including the SWT's video ones which are due to roll out soon). However, that will come and probably at a rate much faster than ticket offices staff are reduced.