This is certainly true for many proteins. The evolutionists smoke and mirrors game is to show a single protein and say that is representative of all proteins. Some proteins are preserved some are not.
Preservation absolutely points to a lack of substitutability in the specified function. How much is open for debate and will be seeing differently depending what paradigm you are accessing the data with.
This is circular reasoning. You first need to establish there are other sequence families and this is true in all cases. Prp8 is highly preserved over hundreds of millions of years and there is no defined ladder for it to climb as it is part of a large protein complex.
I think you have actually helped Gpuccio’s case and we will see if there is correlation between preservation and sequence flexibility.