This sounds good but how does it really help users or developers compared to having a .com website that uses HTTPS? Expecting that users will think "oh, .app, must be secure" doesn't seem like an improvement over expecting them to look for key icons in the browser, or showing them scary alerts for non-https sites.
The only play I see here is that if the new TLD becomes so popular that everyone must have one, then, well, everyone must have HTTPS. But that's not going to happen either. Even .com never reached a high enough level of importance that absolutely every website, including those who weren't interested in providing HTTPS, needed to use .com for their domain.