Of course they will. They'll hold your bitcoins hostage and decide what they should do with them. This is like of you owned stocks and you bought them through a bank. They would have to decide what they weren't doing when the stocks split 10:1. You have absolutely no say in it. The difference is there will be two chains and you may not like what they decide to support!

That's exactly the reason people should keep their coins fully under their own control offline. People not seeing the risk of this are either delusional, or they simply don't care - not sure what is worse. Seriously, how much effort does it take to install a proper client and to properly secure/backup your coins? Security should always be the main priority, but as it is very clear to me, a lot people greatly prefer convenience over security. One should never be at mercy of a central authority, which is exactly why people love the freedom Bitcoin is giving them. Unfortunately, not everyone appreciates that. Essentially, people will only learn when things go wrong (i.e. them getting financially hit in the pocket).