I'm playing Devil's advocate here (I rue the reduction of species as much as anyone), so don't pillory me.
From a pure business perspective I can see Marwell's logic. They have very little serious "large general zoo" competition in the area so they can quite easily get away with reducing species that Joe Public won't miss -gate receipts won't change and feeding, vet and (possibly) staff costs will reduce.
Twycross is in a similar position re local competition and has followed a similar path (Marwell are shedding "just another antelope"* where Twycross are losing "just another monkey" from visitor perspective). An extensive range of hoofstock will unfortunately not impress most of the general public -does anyone really think they'd lose a noticeable number of visitors if they didn't (heaven forbid) have three zebra species?
Personally, I just hope there is some underlying plan (maybe they're taking a step back to make a leap forward). It's entirely possible (in their own mind at least) they're getting everything up to scratch before planning anything really substantial -arguably commendable (if the alternative is to do some good new species exhibit whilst the rest of the park has sub-standard areas, Exhibit A, Twycross).
Re prior comments, I personally think lions (boring to me) and bears (not boring to me) would be great additions in the future in terms of bringing in more visitors. So would elephants or great apes but the investment to do those species well is so substantial (and potentially de-stabalising) I can't see that ever happening.
To be honest I've not visited Marwell for too long a time but should be visiting on the way to the Isle of Wight in the next couple of weeks. I'll try and start from a neutral standpoint and report my opinions then.
*I know they've lost more than antelopes.