Seems to be a bad decision from the beginning.
> And developers really should know better than to fall for the marketing nonsense that anything more than 5 minutes old is "insecure".
Where have you been the last decades? Pick an arbitrary browser and go through their CVE list, e.g.: https://app.opencve.io/cve/?product=safari&vendor=apple
> What bugs?
I guess at that point I have to assume you have no idea about front-end development. Otherwise you would know what a pain in the ass old browsers with their inconsistencies are (e.g. remember the struggle with the early flexbox implementations?).