4 - introducing a new keyword is the hardest thing you can ever ask on this list.

As is already a keyword from with-as and except-as. Perhaps for compatibility “as” is allowed but discouraged as a variable name, and it’s only interpreted as a keyword when followed by an expression and a colon.

3 - Most of the example (and you show it yourself with flask) are already possible with a more verbose syntaxe based on decorators

Yup, a more terse syntax puts the programmer’s focus on the right things.

2 - This will be tempting to use for callbacks and chaining things, leading to the nesting hell we tried very carefully to avoid fom other languages. The example for sorted is also kinda twisted.

I do agree as-do could be easily used for callbacks. Could you please clarify what you mean by nesting hell? Is it better to use as-do where you’d normally pass your own instantiation of a class you create that implements a protocol? It’s always possible to simply pass a bound method of a class you create yourself if you want to keep state.