It depends on where you look. I started my career by diving headfirst into the WordPress ecosystem almost a decade ago now and there are plenty of strong development roles available if you know where to look.

First, someone has to write the software. I'm not sure if you've kept up with the new Gutenberg editor project, but it's a full blown React app and has been for 4(?) years now, with ambitions to take over the entire WordPress admin. Automattic is hiring $$$ for engineers to work on this problem and prototypes that they use internally to build and manage WordPress.com (separate from the Open Source project).

Plugin developers are neck deep solving challenging engineering problems for a variety of sites at wildly different scales. Yoast supports the SEO strategy for the 3 page site as well as the 3 million page site.

Agencies are building sites for huge companies who can't be bothered to bring that development in house. I work for such an agency. We do deep technical work for a lot of big name companies you have heard of. Some of them are even tech companies!

There is interesting work out there, but like so many things, it can often be about who you know to get your foot in the door. There can be a lot of churn, but that probably means the work isn't that great or the pay sucks or a million other excuses that exist with any job. I think Automattic is approaching 2k employees, a huge chunk of them being in engineering. They all aren't working on simple PHP and HTML problems!