I don't see a breakdown of the costs of this layer of hardening. There's nothing on performance, memory usage, library and database support, and so on. Just a few modules that don't work optimally, noticeably the OpCode cache that speeds up PHP quite significantly.

Just stuffing something in a sandbox is easy. Set up a VM and don't add a network card and you've got a near perfect sandbox without any performance overhead. Maybe even do so for each request through Firecracker if you fear memory persistence. What matters is all the added complexity and the downsides of each solution.

Without any comparison, this attempted sales pitch for WASM is little more than a showcase, like when someone runs WINE in the browser.