It is and it isn't. Unity is great but it's got like a min 5MB download for just a spinning cube. Flash (because the plugin was built in) started instantly and had built in support for streaming. Unity can stream, after the 5MB engine download, but for whatever reason it's not common.

I meant more as in people with no technical knowledge whatsoever creating interactive stuff. Just look at the cambrian explosion of indie games from (most) people who probably can't do long division by hand.

Very likely! But some years ago, back when I still played Flash browser games, Unity games didn't work in a browser for Linux, the only platform I use. Maybe that situation has changed? :)

What with a WebGL target for Unity these days, it has. The Unity plugin was deprecated versions ago and they don't even ship a target for it anymore.