Why would he regret it? He got a massive fee for doing what he did and then got to go back to Microsoft after a few years.
I've said it a few times and I'll say it again. WP8 has potential but with Microsoft's glacial development pace it doesn't stand a chance. It can have Instagram, Skype, Twitter, BBM and any other apps but as long as it has stupid niggling issues it will never go anywhere. Niggling issues like a below par keyboard, one volume level for all sounds and a pretty limited notification system.
Yes I know these three in particular will be addressed by WP8.1 but how long after WP8's release will that arrive? They already had a trial run with WP7 and didn't learn. If you ignore WP7 it's still over 18 months before the first real chance of improvements and the chance of some WP8 launch phones being updated is slim to none. That leaves you potentially with people burned by WP7 and then WP8 with no upgrade paths. Do you think they'll return for a third shafting when a WP9 is likely to be incompatible with older devices?