I think you are conflating "alive and well" with "loses absolutely nothing worth caring about from pure classing by multiclassing."
I think everyone agrees multiclassing should be alive and well. The disagreement is over the second (hidden) definition of what "alive and well" seems to really mean and it's dishonest to imply otherwise.
If the design of the game and powercreep and the setup of classes is such that it is compelling (mandatory) to go pure because otherwise you lose too much in terms of DC's and min-max effectiveness, the problem then is with the game designers and not with some imaginary group of "class purist haters who are all irrational and have no reasons."