I'm not sure whether I could follow you... What is the differences of Spring Boot and other frameworks like Micronaut, Quarkus? They all use the same mechanism like DI, Service / Repository, Rich Domain, Builder Patters, Mocks. Just because there are a lot of huge Spring apps we need to maintain? Micronaut, Quarkus are just too new to see some monoliths built with them. And I'm sure you'll get the same problems with them in one huge chaos Monolith 😉

Java apps I need to maintain from year 20xx are mostly based on Spring and J2EE (EB 1.x and 2.x), so in my case J2EE apps are worst...

BTW. this is the power of Java apps using GWT / J2CL, you use the same patterns on client and server: https://bit.ly/GwtIndexedDBDesignPatterns

About Google and GWT, yes, they move everything to Angular but they still maintain J2CL - this is the new version of GWT transpiler (https://github.com/google/j2cl). Difference: J2CL is pure transpiler and not like GWT which also offers UI Widgets, UI development framework (full stack UI). GWT 3 will be based on J2CL. They (Google) also work on WASM with J2CL, just take a look at the GitHub page).

If you want to see an example, here is an example building Web Components with Java (and GWT / J2CL): https://github.com/lofidewanto/webcomponents-gwt-simple-examples

But today you could also start in GWT future-proof. Take a look at this framework: Domino UI, completely Java based: https://demo.dominokit.org/home

Complexity: the mechanism / patterns are all based on the well-known patterns on the server side.

For more information on GWT / J2CL take a look at: https://padlet.com/lofidewanto/gwtintro