Yes, you can run absolutely massive systems with very high throughput and high concurrency using only OS threads and "normal" synchronous programming, if done correctly. However it's generally a pain in the ass because most languages often have their own threading models that may have their own limitations or interaction-with-the-OS quirks, while in C or C++ you're typically on your own for like, all your dependencies including trivialities like thread pools and stuff. And all newer languages tend to instead design their I/O ecosystems around asynchronous stuff. So you have to choose your poison. But handling massive systems always requires a bit of special care anyway.

You will often encounter many other bottlenecks that hold your system back. There are other concerns like latency and utilization to care about. But assuming a purely "compute/IO" driven data plane, yes, you can scale synchronous APIs with threading quite far.