Re: PHP is somewhat responsible for MySQL’s uptake
That is exactly the thing, I think: PHP and MySQL were in the right place, at exactly the right time, just at that point in time when the web was starting to move from hand-crafted static HTML towards interactive web applications, e-commerce sites, etc.
They were both "good enough" back then, and 'word of mouth' counted as a strong recommendation, but it was a long time afterwards before people started to later realise that they weren't perhaps necessarily the "best" choices, but by that time a huge ecosystem had been built up around them (with too often the PHP apps being 'hardcoded' into using MySQL rather than having a database abstraction layer, unfortunately).