Instead if writing a drop-in replacement, which would be a lot of work, the authors wrote a replacement for the workflows they cared about, without nearly as much flexibility or complexity.
A 3rd party rewrite is a great time to assess what features are core features and which are extraneous. I haven't evaluated doas, but I'm definitely in favor of priviledged utilities having less code in general and having less complexity.
Sudo features exist for a reason. People use them. If you delete random features that you don't happen to use, the people who use sudo today won't switch to your new tool. It doesn't matter that in your opinion those missing features are unnecessary. You don't get to make the call.
If your goal is to eliminate unsafe C code from critical paths, you want a drop-in sudo replacement. If your goal is to just be opinionated, sure, make a non-sudo thing with a selection of features you personally consider important --- but don't be surprised if people keep using sudo.
A 3rd party rewrite is a great time to assess what features are core features and which are extraneous. I haven't evaluated doas, but I'm definitely in favor of priviledged utilities having less code in general and having less complexity.