If we do a small retrospective we will see how since the beginning of OS X, the price of the successive updates has been maintained except for the change from OS 9 to the OS X 10 Cheeta edition, with a price of $ 129. The next update to OS X 10.1 Puma was the only one that was free but at the time of ordering they charged you $ 19,99 for shipping and handling and so on to this day with all other OS X updates.
On the other hand, if we rely on the iOS theory, all versions of Apple's mobile operating system have been free since its birth in 2007 and the adoption rate of this system has always been very fast in each version, so we are talking about of a system that provides the latest bug fixes and a less fragmentation.
If we look at the impact and integration of OS X 10.8 a year ago, has equaled the one Snow Leopard had at the time, perhaps this being the most famous version for stability and performance of all that have appeared on Mac. Security updates and different patches are normally given for the last two versions, but Apple has been forced to also include Snow Leopard due to its great popularity.
With all this I mean that if iOS being free the adoption rate of the new operating system is always very high, if they did the same with successive versions of OS X other than, of course, a complete system change (OS XI) would seem better to me because there would be (almost certainly) less fragmentation between systems compensating for the € 19,99 since developers would focus more on looking ahead without having to still be working on "old" versions.
More information - Turn off automatic updates in OS X Mavericks
I'm not sure that iOS was always free, as I think I remember updating the ipod had a cost in the first versions when it was not called iOS yet
Yes, whenever they say that the iOS ones were free, I remember one that for the first generation ipod touch you had to pay, it was only a version
First! iOS hasn't always been free! that was until iOS 3, in fact I paid a long time ago $ 9.99 to buy iOS 2.0 on my iPod touch, at that time they charged it, and I doubt that OSX Mavericks is free!