But we do have people writing some of the most used email clients in the world using HTML and Javascript, so that's hardly a valid criticism in this case.
Apple iPhone, Apple iPad, Outlook, Samsung Mail, and Google Android, together making up well over 70% of that list, are all native clients. Plus, the clients that aren't native are all web-browser based.
So 30% of marketshare belongs to clients built on the browser technologies of HTML and Javascript, the same technologies you claim are "inappropriate" for the new client that we're discussing.