I don’t really subscribe to this notion. It is absolutely impossible to go without the vendors working on custom software development. This is because they not only develop a solution from scratch but also finetune it’s functionality based on specific customer needs.
There are a number of open source licenses available to the users. One big disadvantage of Open Source software is that most interfaces are not quite user-friendly and easy to use. Your options as a user are limited. In terms of functionality and security, you will have to stay within the boundaries of the predefined limits. You will have to wait until the next release.
Sometimes, it takes really long for the providers to come out with a security patch. In simple words, you don’t really have a control over this process. The custom development orders on the other hand often include long-term maintenance and support contracts.
You can call your software provider at any time for assistance. If you are really serious about your web venture, you should consider custom development rather than relying blindly on any open source CMS.