Actually its not just linux but most other open source projects that are funded by major corporations. How easy or difficult is it for a company like Oracle or Microsoft to kill competition? Lets take Redhat as an example. Oracle offers the support for Redhat Linux at half the price for what Redhat does. So is it wrong? Actually there is nothing in GPL that Redhat can do to stop Oracle from providing support at half the price. So to kill of a company like Redhat if another major vendor comes up and starts supporting Redhat Linux at one tenth the cost there is nothing Redhat can do about it. The same goes for most other Open Source projects which are funded by big companies. So if ever Microsoft wanted to kill the competition from major Linux vendors, all they would need to do is to create another company that's indirectly funded by Microsoft. That company can support Linux distributions from other Linux vendors at half or even one tenth the cost of the regular distribution.
Next we come to Ubuntu, as of now its being developed 'cause there is a very strong backing plus there seems to be a lot of excitement about the project. But in the long run once the backing is removed would the project still work the way it is. I am not really sure about it. Right now Ubuntu needs numbers to make a mark and finally a dent on the windows market. Maybe killing Ubuntu might turn out to be a challenge but the the other vendors seem to have nothing concrete that stops a copy cat vendor from coming up and selling the same brand of Linux with another name. In a way the same GPL license might work against most of these companies.