Why would they leave because of small market share? Costs and revenue scale with usage, and Google has near infinite money to invest in improvements.

Being under attack (not just a verbal attack) by the government seems more of a concern. en.wikipedia.org/wiki/Operation_Aurora

From what I understand there was a massive, coordinated, and sustained cyberattack on Google's services that pretty clearly originated from the Chinese government that was interpreted as a pretty clear signal to get out of the Chinese market, along with what you outlined.

Yes. That was the straw that broke the camel's back, so to speak.

There had been a long string of problematic incidents before that which didn't come into the public though. Frustration and anger about China had been rising for years, and there was a very delicate power balance between Larry, Sergey and Eric going on in which Sergey didn't want to be there, Larry was sort of indifferent (if I recall correctly) and Eric very much wanted to be there. So it led to a lot of tension. When the huge hack happened, it ended up tipping things in Sergey's favour. Probably the lack of ability to make progress in China against Baidu, due almost exclusively due to the PRC's playing-field-tipping incidentally and not Baidu's skill, ended up stripping Eric of most of his intellectual ammo.

North Korea also got involved in that action.