From the headline numbers
here, see under "Fiscal Year 2015 Financial Highlights":
" Net new Creative Cloud individual and team subscriptions grew by more than 2.71 million during fiscal year 2015 to 6.17 million."
I've a note from Dec 2013 saying they had 1.4 million subscribers at the end of FY2013, and a target of 4 million by the end of 2015. OK, 4 would probably have been a conservative number. Still....
As for saying artists don't need to upgrade their tools, just use their existing ones better, that's simplistic at best. Sure, the latter is almost always true but it has little relation to whether upgrading tools can make them work better, however better is defined. Some folk simply aren't able to see the new and better techniques that are available, while others will adopt them quickly. Some upgrades have new features that may lead nowhere, while others offer exciting new possibilities.
John