"I suppose I could propose to a client... "I can build your website so that it will look good, work perfectly well, load quickly and validate to W3C standards
Please, do cite a so-called WYSIWYG that will do all that "
I do it all the time with Wysiwyg Web Builder. Notwithstanding that the WWB website itself doesn't pass W3c validator because of the duplicate object ID's. (An easily fixed problem that is *not* the fault of WWB, but the operator.)
"...Makes ugly code which is ugly at the time it is made and might cause significant problems in the future..."
All HTML code is ugly, but how does ugly code make a page develop problems in the future? Using a WYSIWYG editor (or page builder as in WWB), I don't care what the code looks like because when I hit "publish", a whole new page or website is generated. I don't have to know the underlying HTML/PHP/Java, etc, and I don't care.
Note that I did say previously that when a business site gets tens or hundreds of thousands of hits daily, knowing the details of the code to shave a few milliseconds here and there does have benefit to the users. User Lavazza obviously falls into that category and people like him will earn the big bucks that this knowledge commands. However *MOST* business sites do not have anywhere near that much traffic and consider their site a success if they get a hundred hits a day. For those, the few milliseconds that the browser has to interpret extra table or DIV elements that a wysiwyg editor will generate is not noticed. And the site owner will not pay for efficiencies that they will not see nor benefit from.