1) The pace of monetary growth is outstripping the actual usage of Ethereum no one is actually using it they are just trading and holding it in the hope that it will be used in the future
2) These corporations could just steal the ethereum technology and then set up their own private block chain that doesn't even interact with the public one meaning all this hype over corporate investment means nothing
3) It's definitely a bubble that will crash sometime there's too many companies raising too much money based on the ethereum platform with no product when those fail it will have a knock on effect on ethereum trick is to make your P early then diversify and get out/not invest more than you can afford to lose. Tbh though if it was to ever crash I would buy more because it will definitely go up again but if you invest more than you can afford to lose and have to cash out at a loss to pay bills you'll be pissed.
4) If the creator Vitalik gets assassinated or dies its peak because he understands the ethereum block chain technology the most and it needs him to help sort out a lot of scaling issues that are on the horizon as ethereum gets bigger
Some interesting cautious insights here cause most media will tell you that there is no risk at all personally I don't think there's much risk specially right now it's early as fuck but its good to keep this shit in the back of your mind before you start doing stupid shit like getting a loan or running up a credit card.