RealSense products receive tech support on this forum even after they have been retired. In terms of on-sale lifetime though, the past average with previous RealSense cameras has been 18 to 24 months. The 400 Series is different though. As its SDK 2.0 software is open source, it could remain supported by the community of RealSense users even when updates on it from Intel cease. This is similar to the case of the RealSense R200 camera model, where it continued to be supported in the Librealsense SDK even when support in the official RealSense SDK for Windows ended.
There will be new RealSense products, and older RealSense products will be retired. You should continue to receive support for 400 Series products for an extended period after their on-sale life ends though.
In regard to life expectancy, RealSense camera hardware is very reliable. I ran an original F200 model from 2014 for three years without any trouble When problems do occur, they are usually related to software..