If it was so "easy", why wasn't it invented a long time ago?
It was (1983): http://www.billbuxton.com/multitouchOverview.html
Because it made no sense to even contemplate such a feature prior to the availability of capacitive multitouch sensors.
Once the hardware became available, the obvious gesture-based functionality was implemented quickly and independently by multiple engineers.
However, I don't see the real benefit of apple (or others) having patents like this. If they could not patent it, I think they would have included it in their product anyway. It's a cool feature. They got a head start. That's the value of creating something new--a head start. Things move too fast for there to be any real benefit to the general good by granting monopolies for 17 years. People will still create things because you can make a lot of money with a head start, particularly if you're the type of person/company that can maintain that lead by continuing to build great products without resorting to monopoly tactics.
The dropdown menu is a good example where I don't see much difference between a dropdown menu and pinch to zoom.
I have a Ph.D. in an engineering field, so I'm not easily impressed. That said, the inventions which are truly revolutionary are the ones where as soon as someone describes it you think "man, that makes perfect sense".
Yes, it makes perfect sense because it's the correct solution. That doesn't make it obvious before someone came along and spelled it out, nor guarantee that someone else would have invented it earlier.
There are some types of inventions "whose time has come". These will get invented at around a particular time by virtue of the supporting technology existing. Take the brightness control of LEDs using PWM and color mixing of different LED colors to make color changing LED lighting. Philips actually has a patent on this, from 1999. It was an idea whose time had come, and others came up with it independently many times since and frankly probably quite a few times before. But it was inevitable. I think it was a bad patent because it was obvious even in 1999 that you could use PWM, common already, to vary power through a LED to control brightness. It was also obvious in 1999 that you could mix two different primary colors together to form a mixed color. Patents aren't supposed to be awarded by simply taking two already obvious things and adding them together (or at least so says my patent attorney, who is perhaps too ethical about what she'll let me put into an application). But besides all that, I would argue that any "idea whose time has come" is probably also inherently obvious to someone skilled in the art at the time of interest.
On the other hand, there are some ideas which are not limited by any technology; they are limited by the creativity of humans. Take for instance the liquid metal battery that my employer works on. You float a molten anode metal on a slightly denser molten salt electrolyte on a slightly denser molten cathode metal, and bam, super high energy density, super high power density, low cost, long life-time battery. This is, in my opinion, an actual invention. There was nothing stopping someone from coming up with it earlier... but no one did. Why? Because it was non-obvious. It's still simple, conceptually at least.
As far as pinching to zoom? Honestly, I think it's somewhere between these things. Just because it's intuitive doesn't mean it's obvious. It's clearly more of a jump from a zoom button to a multitouch gesture than the jump from "LEDs can have variable brightness and different colors" to "LEDs can be mixed to make color changing lights". But it's also probably an idea whose time had come, by virtue of multitouch starting to exist. But I doubt any of that had anything to do with why it was invalidated; my guess would be that it was invalidated because of sufficient prior art to render it an obvious extension of existing technology.
the purpose of the patent is to prevent secrets from being lost due to the fear of competitors stealing it. Say you have a formula, or a special method of manufacturing a widget. You tell no one, and so stay in business because the widget is useful.
However, in the long run, such behavour is bad for society as a whole. Patents encourage these "secrets" to be revealed, for the price of an enforced monopoly.
If, however, a "secret" method that's being employed is readily and easily discerned by a skilled practistioner in the field, then it deserves no patents. Pinch to zoom is one such "invention" - the mere fact of describing it to somebody means they can implement it.
Amazon's one click buy is also another example.
In other words, if the workings of the invention is easily discernable in such a way that a skilled practictioner could replicate it without knowing what would've been written in the patent, then it deserves no patents.