Yes, Web applications are somehow magically exempt from sound engineering practices.
If you want to make the argument that shitty Rails hairballs that don't scale aren't a problem, because they fulfill a business need and fill it quickly, I'll agree.
That said, that's a business decision, not a technical one.
Every application can benefit from sound engineering practices. Creating a complicated monstrosity when you're not going to need it is not sound engineering practice. At least I don't think it is, but then again I don't consider myself an "engineer."
Once again, Java is a great example. For over a decade, Java developers have piled on more and more "sound engineering practices." Maybe it's just me, but these Java applications are not easy to extend or modify in any way.
If you want to make the argument that shitty Rails hairballs that don't scale aren't a problem, because they fulfill a business need and fill it quickly, I'll agree.
That said, that's a business decision, not a technical one.