Today, even if browsers don't have "View Source", you can search Google for "how to make web sites", and a zillion results would pop up. It's not a roadblock.
My point is, even if IE didn't include that toolbar button, I would've started up FrontPage on my computer sooner or later. It was just a matter of timing and a string of coincidences. Some other string of coincidences could've led me there, too. I say that the argument "some people would tinker and by moving the IDE into an add-on wouldn't let them tinker enough" is pretty weak. If you're curious about something, you tend not to get stopped by things such as that: that curiosity drives you to poke around incrementally until you either find what you're looking for or fuck up the computer (well, at the time, this was something I was prone to doing often because I loved messing with Windows' system files :)
This is simply wrong. I was writing web pages with very little technical ability starting way back in 1996 because view source was built in and not an add-on and that made it easy to learn without any technical expertise. Today the web platform is richer and so it makes good sense for the view source of today to also be richer.