And what incentive is that? Unless you need to latest Xcode to develop iOS apps, I don't see a reason to use Yosemite. I, for one, am happy to have stayed on 10.9 and will stick with it until El Capitan proves to be an "upgrade".
For one thing, they won't sell you Mavericks anymore. If you didn't buy it while it was out, it is simply unavailable for you to purchase. They obviously could sell it to you, but they won't. It's a matter of intentional policy to make sure you don't buy in at any point earlier than the current one.
(Take that in, BTW. That means someone at Apple made the judgment that it is literally more important that you have their latest than it is for everyone to have a stable, reliable OS.)
Overall OS stability has gone down with Yosemite. I have to restart far more often than I should -- usually after coming out of sleep mode.
[0]https://truesecdev.wordpress.com/2015/04/09/hidden-backdoor-...