Taking the case of 47709 as an example, when it does come up for sale all the expensive bits will be missing, so you'll end up having to purchase another in better condition to get the parts. Better off just purchasing the 2nd and letting 709 go.
That was kind of my point David, I personally want to preserve that loco (for the reasons that I stated) but it may not be the best idea overall and could require something else sacrificing itself for parts. This would also apply to the idea of saving 37087...
As for 709 it's kinda sad really as it was so close to overhaul and DRS put the programme on hold. Maybe it will grace the rails again soon though
I'd love to see some 56s preserved and would be thrilled to see a 60 in preservation though from a heritage point of view where would they fit in? Not to mention that a 60 still has a lot of life left (despite EWS' attempts at destroying them) so would be quite costly to acquire.
Modern traction has just as much case for preservation as anything else on the rails, it's just a question of timing. A 66 has no place in preservation right now but in 40 years I guess it would be acceptable for the odd few (since they are the modern day equivalent of the Class 47).
I should also point out that just because something is preserved doesn't mean that it is safe, the East Lancashire Railway had to say farewell to two Class 31s this year as the metal fairies got to them, some other preserved locomotives over the years have also had to be scrapped due to funding issues or the owner passing away and not finding a new home for it. In this case it could be prudent to have "too many" as they will at least ensure that the number of locomotives running in preservation is still enough (unlike so many classes where they are now hard to see).
It could be worth pointing out the case of 56003 which entered preservation and led a happy life, because of it's excellent condition it is now back earning a living on the mainline railways as 56312. If this loco had not been preserved perhaps it would have been cut up instead and looking at the bigger picture perhaps DCR would have decided not to go for 56s but choose to acquire a fleet of other locos instead. A lot of ifs and buts however it's just a point.
Phil