Is this a new, giant complexity or a nice add-on? A huge mental/development shift or a nicely paved new path to something better?
Turbolinks (the project at Basecamp that Hotwire grew out of, now it seems it's called Turbo Drive) was a way to bring single-page style load times to traditional, server-rendered apps. Hotwire is the evolution of that: Turbo Frames let you dynamically replace certain parts of the page on the frontend, rather than having to throw the baby out with the bathwater on every page action. If you're used to developing server-rendered applications without much JavaScript using Ruby on Rails, Django, or a similar "batteries included" backend framework, then you'll be able to add a more dynamic feel to your web app without much of a mental shift: certain template partials (basically the components of your frontend) will be wrapped in this `turbo-frame` HTML tag, which will be slotted in to your page dynamically by Turbo.
I would argue it looks a lot simpler than the average SPA framework or library.
Honestly it's pretty straightforward and the API is tiny. It's great for that grey area between serving server-loaded static pages and full-blown SPA - for example CRUD apps (and lots of things are pretty much CRUD apps). Turbolinks gives you a "poor man's SPA" for little effort (albeit with some gotchas) and Stimulus is fine, although it gets tedious for when you have to work with a lot of DOM elements e.g. "when I click the Subscribe button, change it to an Unsubscribe button". Maybe a less imperative framework like AlpineJS might be simpler.
That lets me have quick development on the frontend, server side rendering for fast first page load and use of a typed language for the backend. The client is also overburdened with a large JS framework.