I know. You can't use most of these "tips" in your monolithic monolith. But you could.

I find two camps: 1. those who need to produce backwards-compatible code, avoiding or not thinking about new features 2. those who can/want to use new features

Camp 1 has the problem of continuously importing kitchen sinks instead of polyfills, which would allow getting into camp 2.

Camp 2 is essentially a luxury of time, thus rare. It already requires mental commitment, which may be scarce after a day of battling bugs for IE.

Simple, elegant solution to a typical problem where everybody else ends up building a broken wheel.

Agreed. This may have replaced Sinatra for my go-to for building quickie Rest APIs

have you tried SlimPHP? I've been using it for APIs lately and its been great. Very easy to get up and going very quickly.