I get the impression making a web app was considerably more difficult for a solo developer pre-jQuery. For a while we had an explosion of choices, and ultimately things coalesced around React and similar paradigms. Meanwhile the per-capita productivity has been skyrocketing as increasingly sophisticated cloud services provide everything from raw key-value storage to turnkey caching and auth solutions.
Is it more complex than single-device programming? Perhaps, but the modern browsers represent a nearly universal platform with very low per-user complexity.
Where the majority of the complexity was in the backend. Now, the complexity has simply moved.
I'm quite happy I don't need to have two different codebases in different languages for most things anymore. I'm more than happy to offload it to the browser to save me some effort, time and money.
You have to validate on the backend among other things. Honestly the only thing we have nowadays is flashier websites for the most part. Some very small percentage of websites actually require the tech that has been built despite the fact that nearly all engineering teams seem to push for it.
Is it more complex than single-device programming? Perhaps, but the modern browsers represent a nearly universal platform with very low per-user complexity.