It would be stupid to release a Series S if it doesn't last the same years as Series X. It would be stupid to have developers having to program for Series S which they plan to not support for 7 years. If at year 6, Halo Infinite 3 comes out and doesn't support Series S, Series S owners will be pissed. If Halo Infinite 3 does work on Series S, but they not selling Series S anymore that would be strange. With that strategy why release Series S, why not just focus on cross gen with One S.
Series S will be here long term as a budget option. Just as Netflix offers 4K and HD options, Apple offer Pro models and Lite models of their latest phones.
The time Series S gets discounted will be the same time Series X gets discounted when we get Series S2 and Series X2.