Its a small fear that data would be lost. The bigger fear is that your workflow is disrupted in a significant way abruptly.
I'm sure Evernote gets a million requests and the company can't accommodate everyone, and maybe Evernote10 is what most of their users want. But for a lot of us, such a radical departure from the past (with no clear explanation as to the scope of that departure) is what is troubling. You don't know what feature you won't have in Evernote10. Further, Evernote6 is officially decommissioned. There will be no fixes nor updates for Evernote6, so anyone who is running Evernote6 is running at their own risk and some of those users, its critical to their workflow. For them, this should be troubling.