> You can't easily spawn 10.000 OS threads.

Of course you can. Go on, compile and run this program:

    #include <pthread.h>
    #include <stdlib.h>

    void *f(void *arg) {
        pthread_mutex_lock((pthread_mutex_t *)arg);
        pthread_mutex_unlock((pthread_mutex_t *)arg);
        return NULL;
    }

    int main() {
        pthread_t threads[10000];
        pthread_mutex_t mutexes[10000];
        for (unsigned i = 0; i < 10000; i++) {
            pthread_mutex_init(&mutexes[i], NULL);
            pthread_mutex_lock(&mutexes[i]);
            pthread_create(&threads[i], NULL, f, &mutexes[i]);
        }
        for (unsigned i = 0; i < 10000; i++)
            pthread_mutex_unlock(&mutexes[i]);
        for (unsigned i = 0; i < 10000; i++)
            pthread_join(threads[i], NULL);
        return 0;
    }

Executes in a second for me, even on macOS.