I'm sure I read on this forum somewhere before that it's part of the agreement of franchises , that they still recruit as necessary.
I don't make a habit of reading franchise agreements but recently I have taken an interest.
There isn't usually a clause to say that a franchise must have X number of staff or need to recruit X many of staff. However, a GTR agreement a few years back did state that they had to recruit a specific number of Drivers. This is actually quite unique in a franchise agreement.
What does happen is that a TOC needs to provide the existing staff counts and that any bidder must enter these into a matrix and show that they have costed all staffing and any future staffing arrangements. This is pretty much standard across all franchise agreements and invitations to tender.
in reality, the recruitment process never stops. The day to day just continually ticks over. What is more important to watch is what happens when there is large infrastructure and timetable changes. GTR and Southeastern just had a major infrastructure project finish (or at least should be finished) that required a major internal change to staffing, depots, training, stock etc etc. This had to come with a huge overhaul of staffing. GTR has had an impact at Southeastern.
MTR for Crossrail had a recruitment drive recently and that will have had an impact on the TOCs local to them, especially where they initially toc part of a TOC and TUPE'd them over.
If Northern do lose the franchise, I really wouldn't worry. Everything will tick along as normal. If the franchise splits then I would be thinking about it and may have a small concern as there will be a split in depots and that will cause some kind of internal shift with which staff would go to what part of the split. There would probably be a pause and a complete rethink of staffing levels and and potential candidates would get bumped to a different depot. (a bit like hat happened in the epic GTR recruitment thread)
Potentially this is where a talent pool may actually be a good thing. A TOC is more likely to hold back on posting any new vacancies and dip into their talent pool because it would save time, and money. It would no doubt be easy to reflect numbers when a franchise changes as you have existing staff+talent pool as a fixed number.....
...not forgetting the Williams review :/