I don't know that soft sand is such an issue and warrants a lighter bike. It just needs a different skill. I used to hate it, until I went to Fraser Island (largest sand island in the world in Australia) a few times and followed some direction.

Taken a 3AJ Yamaha, BMW Dakar, DRZ400 and a WR250R. Once you get the hand of it you'd take anything.

Keep momentum, balance and constant throttle, and your arse well over the rear axle. Then steer with your feet, not the front wheel. You don't have to go very fast, but your bike will like it more. The front wheel becomes a plow, and the bike will get very hot. When you loose my balance it's common to pull on the bars to regain it, by which the front wheel will hit the side berm (in the case o a sandy trail). If you do that quickly, you come off. Instead do that slowly, by backing off the throttle without turning it completely off. Your bike will still go forward but not in the direction your wheel is pointing, because that's pointing into the berm. With the right speed (slowish like a trials bike, but constant) the sand resistance is greater than the momentum into it from the front wheel, and you don't push through it. Then just re-balance, point the front wheel into the direction of the trail and slowly built up speed.
It's all about smooth and slow. I'm actually looking for some sandy trails to build up confidence again after a big crash. It's a great place to learn to throw the bike around. Relatively low consequence. Particularly compared to hardpack dirt or gravel.

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