Good on the Duke, for getting to be able to look at all this so-called mimic experts - the same people that he knows have always looked down on him a little bit for being a mere enthusiast. Some knob with a *hobby* - and being able to sniff your nose at them and say "How strange, I'VE never had problems dealing with half-feral mimics." Must be a skill issue.
The Duke of Boxford won't fault the Steamworks crew for being cautious. This was the first time any of them had seen a half-feral mimic, and while Hiro had been compliant, Spencer had been acting out aggressively until his owners calmed him down.
It hadn't been looking great, and they did what they thought was the safest option.
The Duchess just didn't feel like it was the right thing to do for Spencer, and the Duke believed her when he saw how frightened their mimic was.
He does, however, fault the experts outside of Sodor for never really figuring out what feralization really is.
Seeing as he and his wife essentially rescued Spencer when he seized to be useful and was to be put down with the rest of his siblings, it makes sense to him now why he had so many behavioral issues and why he went half-feral.
How many mimics died because they'd been put through traumatic events and stresses that they simply hadn't been able to cope with? How many mimics had died because no one realised they were susceptible to mental health crisises that affect them in such a drastic way? How many mimics died because so many of these "experts" would rather believe they're dumb and replaceable animals?
Needless to say, anyone on the mainland who looks at Spencer wrong, is going to be met with suspicion from there on out.