One of the major reasons for that position is that OS X updates are now free. The user experience comes first, no compromises. But please note that I'm not say

One of the major reasons for that position is that OS X updates are now free. The user experience comes first, no compromises. But please note that I'm not saying that I'll drop support for 10.9 because there's some convenient API on 10.10 that will save me a few days of work. 10.10 would be a requirement if and only if it provides a meaningfully better experience. I also think some developers might start requiring 10.10 for their next major versions, assuming they have been designed in line with Yosemite's new visual language. Supporting both < 10.10 and >= 10.10 UI-wise would involve non-trivial amounts of work, would for most small indies would be impractical.