What exactly is the argument that we're having?
If somebody were arguing that a website loses value by virtue of having been made in PHP, all else being equal, you're right, but nobody's arguing that.
If somebody were arguing that PHP should fall off the face of the earth, you would be right to say that this is an awful argument because so many people are already used to PHP and are productive, and learning a new language takes time. And I think not many people are arguing this either.
The relevant argument is, for the sake of maximizing the potential to Build Something Successful that People Want, PHP should rarely be advocated, because better languages and platforms exist. If you come back to me by pointing to Facebook and Wikipedia, I would ask you to explain to me how, given the challenges leveled againste PHP as a language, it could possibly be a preferable language to Python or Ruby.
The only redeeming value of PHP that I can think of is the easy setup. And I don't think that would help Facebook or Wikipedia if they started today (I'll grant you that PHP may have been the right choice given a lack of web frameworks at the time they started, I actually don't know why they chose what they did).
So by this reasoning, unless there can be a list of benefits to PHP that would lead you to believe that it would help lead to Facebook's and Wikipedia's success if they started today, the rest of us are left with the conclusion that, based on these ridiculous deficiencies, there's no reason it could possibly help them and it would probably hamper them. But more importantly, it's a losing proposition to learn and use to start a new project, other than something that's starting small and needs to get off the ground really quick.
Is that unreasonable? Imagine everybody was "doing just fine" making successful websites in assembly? How about a realistic case study: Ok Cupid wrote their own webserver in C++. They still use it because it's not worth redoing their whole infrastructure. But if they started today, they would have used Apache. They don't advocate anybody, themselves included, to write a web server in C++ (edit: today), despite the fact that they're "doing just fine" with it.
So if somebody is criticized for using PHP, and their response is, "Yeah, I know, but it's what I know" that's perfectly fine, learning a new language takes time and it may not be worth it, it's not my place to make judgement calls like that for people. If their response is "Hey man, I'm plenty productive with this, to each his own" imagine if they said that about assembly. Yeah, I guess they could be wired in such a way that it's more productive for them, but it's got to be such an oddball case that it doesn't justify the existence of the language. Yes, part of me wants to live and let live. But I can't sit back and accept this subjectivist attitude toward languages, there's actual reasons a language is good or not.