Played TSPP and GOTG yesterday for the first time and both were fun. GOTG had LED screen and more but somehow TSPP had way more magic to me with a cohesive package including original music and voices etc. Just had more pinball soul somehow - it's not just about more and more stuff, it's about how It all comes together. WPC had this in spades.
So my point is that even comparing Stern vs Stern I would say they are not necessarily getting better and better over time. And your list of Stern successes backs that up - mostly their older games.