> A concurrent thread being allowed to jump in and modify memory at any time at all, even halfway through a primitive math operation (the deranged C / Java / Python[1] memory model) means a correct program is a morass of locks and defensive programming and an obvious program is horribly broken.

This ignores that most threads, most of the time, operate on their private memory. When your request thread is querying the database and rendering some HTML based on the data from the database, then the only thing that needs to be guarded is the database access. The HTML rendering happens on thread-private data.