The teething problems with the LS410 motor have been sorted. RE isn't going to discontinue a motor/platform that out performs their very own 500 motor, in every way.
Well, I certainly hope they don't drop the 410, I agree that a 650 (unless it's a 650 single on similar cases) would make an already pretty heavy bike way too heavy for my tastes, and outside North America's Interstates or the autobahns, the 410 will do what most people need it to.
Having said that, it doesn't outperform the 500 in every way. Here in the Indian Himalayas, last week I rode both the EFI Himi and a Thunderbird 500x (also EFI), within a day or so of each other. I've also ridden the carb'd Himi and carb'd Bullet 500 UCE side-by-side, and the 500's simply have a lot more torque - friend and I were switching off sharing both, and we both agreed. The 410 would probably catch up / keep up at higher rpm's, but it is not as instantly responsive, it doesn't push you back in the punchy sort of way the way the Bullets do. My modestly modded AVL500 also pulls MUCH more strongly. IMO the Himi is an interesting bike, a good-looking bike, a kind of reasonable all-rounder, even a good-sounding bike (newer version anyway) - but it is a rather anemic performer, at least at 7,000ft and above. Besides that, carb'd ones, at least, allegedly manage "only" around 25kmpl in the hills, whereas the 500 does at least 30-32. Besides that, both versions of the Himi I rode still felt pretty unrefined in the engines - the vibes are there (though higher frequency than the Bullets, it was enough to make something rattle very irritatingly in the instrument cluster), the slightly clattery valvetrain is there (500's have hydraulic tappets), the deeper down sounds (piston-slap? Crank?) also were there under heavy throttle too - and neither bike was a high-miler.
Other thing, apart from engines, is that it really doesn't have a lot of suspension or ground clearance... If a 650 comes, they cannot afford to make it any worse in that regard. A Bullet with a high scrambler/woodsman pipe down the side would allow the brake lever to be raised, in which case it would have about as much ground clearance as the Himi - at least with someone actually sitting on it. Despite the Bullet's lack of suspension travel (5.5" up front vs. the Himi's 7"), they actually aren't too bad on the rough tracks, they don't settle down through 2/3rds of the travel when you sit on them, the way the Himis do. I've seen expert riders on the Ladakh road pushing the bikes'/riders' limits side-by-side, and was really surprised to see that the Himi couldn't really get ahead of a UCE350 (Electra) out there.
You find a lot of people raving about them, there are the world-tourers / trail-riders who sound convincing, etc, and they're a pretty good value, but honestly I'm not overly impressed personally. In general, though it might be the better balanced (or is it more compromised?) than either, I'd prefer my old DR350S or AVL Machismo any day. Either is much quicker / more responsive, and the DR's suspension is to die for.
-Eric