Fair. „Dreißig“ is quite an outlier.
But wouldn’t it be boring, if the rules didn’t have exceptions?
But also, the way our constant writing reforms are going, I wouldn’t be too surprised, if, at some point, „dreizig“ becomes the correct spelling. In some dialects, it’s already spoken more as a ‘z’ than a ‘ß’.
Then again, I’m still mad at how they massacred the spelling of reflection.