It's not like you are always writing better code than the open source projects are. Unless you are one of the best developers in the world, then sure, then that might work, but for the rest of us, we are probably not guaranteed to ever write code that is 100% bug free for five years.
Security vulnerabilities grow in unattended code then?
Or they were there from the second the code was written but with some luck someone noticed them and fixed them?
Old code isn't necessarily insecure just because it's old...
You want me to believe that in the npm "ecosystem" they have LTS branches that only get security updates? For anything besides maybe a few large libraries with companies behind them?
Compare that to something like jQuery where all the edge cases have already been accounted for 10 years ago.
People who don't "over engineer" also don't track errors. So you might be losing a bunch of users because your website is broken on their devices, and you'll never know. You can't repro it because your locale doesn't match theirs.