This is unquestionably a game changer and one of the biggest announcements at the keynote. This effectively levels the playing field with Android at least with respect to functionality, and it allows iOS to really act like a PC replacement. This is what iOS should've been all along.
Not sure I agree with this. If apple had allowed infinite extensibility from the beginning, who knows what bugs, stability issues and vulnerabilities would have been manifest, irreparably harming the reputation of the nascent platform, not to mention, how slow and incapable the hardware was at the time. Those who wanted that flexibility could jailbreak. It's not like they had any trouble selling phones.
Apple certainly could have but chose not to, deliberately. Now they are doing it in a way that will compete quite well with the Android platform, from a timing and marketing perspective, without any of the well known issues, giving Apple a distinct advantage.
This has been planned for quite some time, and from a strategic and technological standpoint, it's a brilliant move. I think they know what they are doing quite well.
What I find interesting is that the people who REALLY want to tinker, who actually KNOW how to tinker i.e. DEVELOPERS, now have a set of powerful and vastly superior set of tools at their disposal, vs Android which really is a bunch knobs to play with until you are bored.