Maybe. But knowing RM which would they rather cull? Senior posties with better contracts, aches/pains/limps, doesn't mind an argument with a colleague or manger, maybe even doesn't drive or drive for RM? Knows what "cutting off" is and how to do it properly?
Or a younger person that is on a new contract, can drive for RM (better for P&L/LAT/nip off to do a 9am Special/Sunday shift/now Saturday only parcels maybe??). And they don't feel the job at the moment and bounce into the office in the morning, just do what they are told to do without comment? Think they must stay out until the van is empty? Hmmmm.
Newer staff don't have the knowledge and I've seen them doing loops near the DO looking slightly lost. Probably can't find the house number and perhaps only vaguely aware that the current letter in their hand is a mis-sort for another loop or duty (e.g., they are on X Street and the letter is for X Road which is the other side of town) and they should just move on to the next letter. However, RM will put up with that and are probably happy losing all the knowledge about where you can leave things, or that number 12 will take a parcel for number 8 (because they are the same family), or that number 21 works at the shops round the corner and you can drop it off there (or are we all in the mode of "deliver as addressed!"). It'll be mayhem but whatever.
I don't think seniority equals security with what may be coming round the corner.