There are tons of changes under the hood that made webapps development easier in the past 5 years, and not all of them can be polyfilled. For example, ES6 Proxy

There are tons of changes under the hood that made webapps development easier in the past 5 years, and not all of them can be polyfilled. For example, ES6 Proxy is something an end user won't even notice it exist, but now used at fundamental level on many webapps. Developers that depend on it won't trade their productivity gain from using the new api with supporting browsers that has less than 1% marketshare.