We have open sourced 95% of our code, which gives people a really great product they can install on their own servers. Along with our hosted version, we have a premium self hosted version with a few things that only the big users would want. I would not personally want to build my business around support/contracts as it is really pretty close to being a contractor.
We were already an established company before we open sourced so I can not speak for how many users it got us. I don't think its foolish todo but at the same time it is not going to make your business.
Be aware open sourcing will add extra burden to everything you do. But starting open source is going to be easier than doing it retrospectively.
discourse.org started life open source and they are doing very well but have some big hitters and finance.