How many in-order cores could one fit on a die?

Several times more. Sun Niagara T1 had 8 cores X 4 threads = 64 threads, at a time when the complex OOO CPUs had 4 cores max with 2-way SMT (8 threads).

That machine got considerably more throughput on highly parallel web workloads as a result (as long as you didn't run a web app in a language that indiscriminately used floating-point, like PHP, cause they gave it one FPU to share between all cores!).

See link in another comment to a blog post with more details and references to a couple of really good papers - old, but still good reading.