The reason why some users might be put off by this wallet is because in general, you don't even know whether someone is using Proton Mail for their email addresses without looking at their domains' DNS records.
The average person does not know what a DNS record is.
So they might try to send some bitcoins to a user with a Gmail address and a custom domain, and when that fails, they get frustrated.
Ι guess that their software would require - before sending - that they search to see whether the email they 've put accepts bitcoin on their email.
So, the "send to email" thing should be a software-related feature.