M4xim1l1ano For what it's worth (and because people seem intent on missing your point and piling onto you), I think you might be right-ish in your thinking.
The Series S is just "That other console but cheaper and worse". There are no up-sides to buying one other than saving a couple hundred bucks. As we have seen from countless market data, people seem to be less price-sensitive in big purchases like these, as long as there is (perceived) value there.
A portable Xbox would have appealed to a different segment of the market, and yes, could have made people double-dip.
It would probably have taken on a non-conventional form-factor, given the power-requirements, but that's the sort of thing you get your creatives and engineers to think about, and also task your marketing team with probing the market to see what people might be receptive to.
Would it have been a success? Well I kinda doubt it, but nobody will ever know because they never tried