We're beginning a new site launch project, and currently in the midst of assembling our strategy around the Javascript architecture.
After so research I opted to vote for using Webpack and BabelJS. Our tech lead is concerned that our team isn't ready to use "advanced javascript" and it will cause issues.
My answer was that ES2015 encompasses a lot of additions to the language, and that we don't have to use all of them. Things like fat arrows would be a quick win. We don't have to start using Classes unless we all agree that it's a good idea. This didn't really seem to sink in though ufortunately.
We're going to have to maintain this site for 4-5 years, and in the near future we'll be required to read/write ES2015 style code if we want to develop apps in React, Angular, etc. because their documentation will be using the new syntax. Backporting things sucks, and will cause more problems than its worth.
These points didn't really seem to sway his opinion, so I'm hoping to get some outside feedback on how you've pitched this topic to your teams and won their approval.
You are outlining the benefits, but JavaScript is very different from other languages and can be hard to understand, let alone master all of the libraries based on it.
If you can address those issues, then you may be able to convince your lead.
I've been on both sides of the coin here. When deciding on a platform or language, I usually choose what is going to be the easiest to use, or what has a high adoption rate which will provide the documentation and Q&A information online.