So what's the problem with extending regexp then?

It seems pretty clear from reading the above thread that burntsushi is saying that they have not been able to find a way to implement the extensions you suggested without making the compile time unfeasible for general use. Given how prolific their work in writing regex and other text-search related libraries is, it seems reasonable to take them that their word that they have in fact spent time trying to solve this problem. On the other hand, it doesn't sound like there's any any practical implementations that anyone can point to that can in fact add these extensions in a way that doesn't cause the compile time to be infeasible. Given that, you seem to be making the stronger claim, so it's a little strange to put the burden of proof on the other side of the argument.

I already told you. I'm done wasting my time speaking with you in this thread. In particular, you seem unwilling to acknowledge any specific trade offs that I've pointed out.