I think the biggest advantage of 650B for new bikes is that it lets you do things with the geometry that you couldn't do with bigger wheels, especially on smaller frames. For instance, 700x40 tires require longer chainstays than 650x42 tires. Obviously you lose a little air in that transaction, but for most gravel riding 650x42 are plenty comfortable.
Personally, I'm in the camp that's using 650B to retrofit wider tires on an old road bike. I recently built up a 1974 Motobecane Grand Jubile with 650x38 Panaracer Pari Motos and a 10-speed Campagnolo drivetrain, and just to be sure I was in full 30s/40s randoneur re-enactment mode, I raked the fork a bit to give it low trail. It's a very fun bike to ride on gravel, but possibly not quite as well suited to it as some of the best modern adventure bikes.