Multi-Threading is the worst solution, except for all the other solutions.

Avoiding multi-threading doesn't remove concurrency issues. It just moves them to a different point in the application execution. A point where you don't have debuggers and need to create overly fault-tolerant behavior for everything. This is bad for performance but worse for debugging. With a regular synchronous threaded application I have a clean, obvious stack trace to a failure in most cases. Asynchronous or process based code gives me almost nothing for production failures or even regular debugging.