Earlier quoted context omitted.
Tech lead of Google Registry here. I can help answer some questions. HSTS preloading offers the highest possible level of security, as the user's browser is enforcing the use of HTTPS. Merely serving via HTTPS is only optional security, as any man-in-the-middle attacker can strip that encryption (see sslstrip, released six years ago). For more information see my blog post from last year: https://security.googleblog.c…
I know TLD's doesn't affect SEO, but does Google enhance .app domain SEO when people search for apps on Google search?
So you have your answer.