We all know 360 was a late bloomer, so Series should stay ahead for at least the first 3-4 years.
It is way too early to say if the Xbox Series X|S will outsell lifetime Xbox 360 sales. But with its stronger lineup going forward I think it is entirely possible. As long as the current momentum continues. Xbox 360 did get a huge boost from Kinect and I don't see anything coming to the Xbox that would cause such a boost in sales that late in its life as Kinect tapped into the casual market.