From a common sense point of view if, for example, the B747 was to be scrapped why would a company want to make the most junior pilots on, say, the B777 fleet redundant and then go to the expense of conversion courses for all the former B747 pilots?
Alternatively, it would be cheaper to retrain and keep junior 747 34pp scalers rather than the more senior 777 24pp scalers.
Let's hope that can of worms doesn't need to get opened.