I'm too lazy right now to dig out old buyer's guides. But off the top of my head, and using 1990 as a reference point (that seems long enough for a "long, painful death"), here's some impressions:
Ducati - still building them, and I suspect selling more than in 1990.
Buell - definitely selling more
Moto Guzzi - dunno.
Suzuki - had no V-Twin sporter in 1990, IIRC. Selling loads of SV-650S's now.
Kawasaki - none in '90, none now, for a net decline of none.
Yamaha - ditto
Honda - I think they might have had a 250 V-twin back then. Got the RC51 now. Bet that's been selling better.
Aprilia - none in 1990, several models now. In a classic display of kpaul's patented "Absurd-Unwarranted-Conclusion-From-Evidence-Presented Misdirection Ploy", there is nothing in his link to indicate that Aprilia is abandoning the V-Twin models (in a related story, kpaul reports that Coke has introduced Vanilla Cherry Coke, signalling the death of regular Coca-Cola.)
The idea that a brain that works as illogically as yours is apparently involved in the production of commercial aircraft is frightening.