You mean in a single-threaded context like Javascript? (Or with Python GIL giving the impression of the same.) That removes some memory corruption races, but leaves all the logical problems in place. The biggest change is that you only have fixed points where interleaving can happen, limiting the possibilities -- but in either scenario, the number of possible paths is so big it's typically not human-accessible.

Webdevs not aware of race conditions -> complex page fails to load. They're lucky in how the domain sandboxes their bugs into affecting just that one page.