Good engineers will always be paid extremely well, regardless of what their domain is. Web dev is competitive at the low end of the market, but certainly not so at the high end.
This is sort of naive. Two truths: (1) Companies have a hard time finding people with decent CS knowledge, and (2) you can get to the top (or near it) of the web dev field with little to no CS knowledge. This is why you hear the chorus of "I've never ever needed to use/think about $CS_TOPIC" when this discussion comes up. The upper end of web development is hyper-competitive and trend-driven because CS knowledge is an insufficiently differentiating factor for productivity and work quality, which means that anybody with good intuition and memory can reach the top quickly.
> Companies have a hard time finding people with decent CS knowledge
Agreed.
> you can get to the top (or near it) of the web dev field with little to no CS knowledge.
That depends on what your definition of top is. A person with little to no CS knowledge won't be working for any of the big 4. They won't work for any of the top tech firms because they will never make it passed the tech interview.
Unfortunately the term "web developer" is fairly broad. Its so inclusive that a designer that can struggle their way through jquery will frequently be lumped in with an engineer at Facebook working on React.
How much are the big 4 paying those of us that don't have hard jobs?