Johnny assumes HTTPS Everywhere automatically switches sites to HTTPS when available.
Probably
So when he hits a login over HTTP he shrugs and says "I guess they don't have HTTPS" and fills in the login anyway.
Maybe he assumes that that page is not aviable in https. But if he transferes a password unencrypted, that is his fault, and his fault alone.
Johnny realizes that more and more, with HTTPS Everywhere installed he doesn't need to worry about the lock icon in the URL bar.
No. Why would that happen? It's still there, still very visible and just as important. If you installed https everywhere I'd assume you care about this more than the average user.
After all, if HTTPS is available HTTPS Everywhere will automatically switch him over, and if it isn't, there is nothing he can do about it anyway.
This is true in most cases. But assuming that this makes it less important is just wrong.
Are webmasters really going to jump through hoops to make a ruleset for HTTPS Everywhere, when it's probably easier for them to make their site HTTPS default (and use HSTS/HPKP etc) which help everyone (not just users of a specific addon).
HTTPS everywhere is not a tool for webmasters. It's a tool for users that are ignored by webmasters.