Interesting mindset, as if a non operational locomotive is comparable to a dead animal?
Very few people are enthusiastic enough to track down a certain locomotive at a running day at a preserved railway. They're most likely to experience it at a museum. If people have no interest in
"stuffed animals" then why would they travel to the NRM? Where there are usually only static exhibits?
The NRM is being talked about as if it is some shadowy entity with malicious motives, but their strategy isn't exactly a secret, they publish it here;
When loaned out, locomotives experience wear and tear, and suffer damage. In some instances they've been badly neglected (see the entirely avoidable decline of the LEV1 for example!). It's often not in the best interests of the locomotive or item itself to be pressed into operation.
Added to that, objects on loan aren't available for visitors at the museum. They're only available to a smaller number of people on a different site, and only for a fee, on days where they're operating. The NRM's aim is to preserve railway history and tell the story to the public, to educate visitors about the history of the railway. The trains on display help to tell this story. If all the diesel locomotives at the NRM were considered to be operational and not put on static display then it would be very difficult to tell the story of the railway since the end of the steam era.
I think this illustrates the NRM's argument rather well.
55002 is not the only Deltic in operation. There are 5 others which have been used on the mainline in recent years.
But, because the other five are intensively used and experience wear, 55002 does happen to be the only one that isn't either broken or undergoing heavy maintenance. Hence the argument that that because the others are broken, 55002 ought to take it's turn to be used, probably until it is broken too.
OR, the NRM can keep the status quo and present it as the only example of it's type on static display, while also being maintained to the highest standards.