Aha, now let's look at this from a mathematical point of view:
First, shipping must be well defined. We will define shipping explicitly as it is generally assumed: The canon revelation of mutual attraction between two characters.
What exactly is it meant by Negaishipping having the same chances as Cafemochashipping? Define N as the probability of Negaishipping and M as the probability of Cafemochashipping. You are claiming that M = N. Therefore any values we find for either M or N will respective go to the other unconditionally.
Let's assume the argument that no or none of these two ships are possible, N=M=0. The only problem is that we never know the chances of a ship actually becoming canon. You are not a writer, and therefore you cannot claim a 0% chance on anything.
However, the argument that the probability for the main ships decreases for each passing generation cannot be refuted. So we can assume that it is possible for N < Pe < A < Pk, where the later implied are Pearl, Advance, and Pokeshipping respectively. Your argument would therefore include the possibility of M < Pe < A < Pk.
But now reasoning must be thrown into the mix. Homosexuality is a taboo in a show with an age range of the tween years. By how we have defined shipping, it is a taboo for Ash and Cilan to canonically have mutual attraction. This can mean two things:
M = 0
M = lim_x->0(x), where this means we can assume an infinitesimal probability.
But of course, M = 0 is false by our argument of "We are not the writers", so the infinitesimal chance remains.
Therefore, through your argument, N = lim_x->0(x) as well. However, the possible argument that probabilities of later main ships are decreased brings a mathematical and logical inconstancy. Let A represent the next ship with Ash and the next main girl.
If A were also forced to have the same infinitesimal chance, this would obliterate the assumed argument. Therefore, we cannot assume of any ships involved with that inequality any more, therefore nothing can be assumed of N anymore. Therefore, M = N cannot be proved, therefore cannot be stated.
If A were forced to be zero, this would show that either the generation of A would be the last generation, because the argument cannot have negative variables because they represent probabilities. Or the argument would once again shatter, leaving the same aforementioned conclusion of the stateability of N=M. This assumption would contradict the "Writers" argument in the first place. And by contradiction, N =/= M
In conclusion, N=M cannot be stated, or N=/= M. It is most possible that N > M and M can be assumed infinitesimal.