Well, there's a loaded post on a stroker forum!

From what I can see, not being a 4 stroke hater and all that;

Derbi and Aprilia (along with KTM and Gilera) race what they can make in a reasonable budget and what they historically have had market success with, GP wins, titles and all the kudos that goes with it is what they are chasing, the engine is irrelevent to them, Aprilia don't even use their own engine in their GP bike yet they can plaster "multiple GP World Champions" across their bikes (and scooters), adverts and literature etc.
They have also have tried and failed to make a MotoGP bike, huge money to build one, huge money to run one and huge money to hire a guy who's likely to put it at the front ensures the likes of the small companies will never succeed

The "modernization" of the 250 class (400cc 4 stroke twins are scheduled to be introduced in 2012, 2 strokes will be made against the rules 3 years later) is already in motion and is indeed to give it market relevence and attract the other large Jap manufacturers back to the class.

They scrapped 500's for the same reason they're scrapping 250's, market relevence, manufacturer pressure (Honda don't like two strokes, Kawasaki don't build them, Yamaha's effort was minimal and Suzuki's was worse) and possibly enviromental issues.

As for bigger two strokes, no chance, the 500's really weren't much (if any) faster than the 250's despite having near twice the grunt and they were instantly left for dead by heavier and barely more powerful first generation 4 strokes. Anyone who's ridden a similar sports 2 and 4 stroke will tell you it's usability not power that is the 4 strokes ace card as far as fast laps are concernd.
All that stuff the racing fans of the late 80's and early 90's loved (basically razor sharp strokers with rubbish tyres throwing all but the very best up the track at every given oppertunity) is the stuff that engineers spend years and millions of yen trying to get rid of.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread