Well, there've certainly been a number of TriShaws rumbling past BEagle Towers during the past few days, so I guess 21s 6d are busy again!
I expect in a short while that situation will change...the RAF has never been noted to be able to keep them flying for very long, without major snags.
Their operational record (such as it is) speaks for itself.
Technically, the TriStar having 4 hyd systems vs DC10 3 might have looked better but the Sioux City deal would have taken the 4th system out as well, had it been a TriStar.
Don't know much about the TriStar, do you?
Due to specific number two engine placement, loss of all hydraulics in a Souix City type of situation is highly unlikely.