Yes, for JVM, it's not that trivial and non-blocking code is definitely not mandatory. Our app backend has been written in a blocking fashion and we are gradually transitioning to non-blocking code.

Your question is very valid as the JVM can handle even thousands of threads (given the host OS can).

Having less threads does keep memory usage lower (as each thread in the thread pool has its own stack space). Also, it's more scalable because if using blocking code execution, you'll need to have one thread per concurrent request served. Lastly, thread starvation can become a problem with blocking code in case of delayed I/O or other failures. If that happens, the server becomes unable to serve new requests.

See this SO answer: https://stackoverflow.com/a/63490797