People need to understand the reasons why we are where we are with this. If it was more economic for the TOC/FOC (anyone at all who pays overtime on a regular basis) to be fully staffed and not want to make overtime payments, then there would be a lot more companies around aspiring to this goal. The fact that there are not gives you the answer you need.
There are quite simple economics in progress. For every member of staff you employ you take up administrative time in the recruitment process, often these days employing agencies to whittle out the favorites prior to formal interviews. External applicants will take up HR staff time, internal applicants line managers time. And so you are on the books, welcome to your new company and we are now paying you to be non productive for anything from a few weeks to a good few months. Training courses to attend, run by another agency or internal staff. The cost to a TOC employer just to get a person booking on for their first full day of duty for the lowest graded staff is thousands, go up into management and driver grades (signallers are similar) and you don't get much change out of 30 - 40 grand.
A lot of people think Sundays should be part of the working week and RDW eradicated. My roster pattern allows for 2 x 12 hour Sundays every 4 weeks. There are 4 of us on the roster so that is 96 hours every 4 weeks that now needs to be covered by somebody else. Remember I am still doing a 37 hour week, these Sundays are extra, so I can still do my normal job but you need to employ 2 and half more people just to do my grades Sundays. There are six more grades in my office (just my TOC) with identical shift patterns, So you will need to employ an additional 18 members of staff in total to eradicate reliance on overtime.
Now continue to pay their NI, Pensions to sort out and suddenly a few RDWs / Extra Sundays with existing staff appears to help the balance sheets. Other commentators have recently said the staffing costs to a TOC pail into insignificance compared to track access charges. That may be true, but the railways have been privatised for a very long time and I see little evidence of any TOC attempting to staff up to full 7 day a week working with no overtime. There are a few exceptions I know, but LM were led to the situation because the staff were not doing the overtime and they now have a mish mash of staff on old contracts still doing overtime and new staff on 7 day contracts. I am guessing that is a nightmare to roster !