This can be proved by induction. Or you can cite Craig's theorem (the less known one) for that. See [1]
Honestly, I don't see the endgame here.
[1] https://math.stackexchange.com/questions/839926/is-there-a-p...
I see the endgame now, thanks guys.
Consider a computational model that, rather than work by successively rewriting an expression over and over in a way that honors some equivalence relation over expressions, it works by explicitly building the sequence of such expressions. In that kind of system, every computational state properly contains the previous state. Things grow and grow and never get "deleted". Yet such a system can clearly be universal.
I wonder how long in advance Stephen Wolfram first had this thought and waited until the centennial to publicize the suggestion.
In the negative case, it would say the idea doesn't pan out.
In the positive case, it would mean that you can use just S instead of S and K when doing combinator reduction, but doesn't change that this kind of reduction is not super efficient practically speaking.