As for the validation, well yes and no. I don't think many people would argue that it's impossible to upgrade any parts on the SL, in fact on ANY bike short of a MotoGP prototype.
What you, and most other people cannot do that the OEMs can, is to develop, test, and validate all those parts together as a package. The point you may be missing is that for some people, there is value in that rigorously developed OEM package.
That's true for a "normal" bike, which is why you can upgrade the .... out of something and never recoup your investment when you sell it. And it's doubly true for a collectible/limited/rare bike like the SL. Throw whatever parts you want on it, to a lot of people you've just made it worse.
But, so what? You don't need my or anyone's validation, just do what you want and be happy with it.
If I can offer my $.02 though, I think the reaction you received might be a result of your own approach. From what I have read of your posts, the general tone of your reaction to the 1299 SL was basically, "what an overpriced POS, Ducati didn't even give it x,y, or z, I can build something better for less money, you'd have to be stupid to buy one."
I can't imagine that would come across too well to a new or hopeful owner of the bike, or even somebody just admiring it. And being in the industry myself, I can say that you don't really know what's involved in OEM bike development and are just armchair quarterbacking.
Not saying that you can't or shouldn't criticize the bike, go ahead. But I think that doing it in a way that comes off as less "superior" sounding might get you a better response to your own project.