I disagree, and I say this as one of the maintainers of a decently popular open source project (https://www.fluentd.org)
It's true that open source allows you to tinker and that it lets you jump start your project. But as you point out yourself, a huge part of your software cost is maintenance, buy or build, open source or proprietary. The reason why open source is great is that as long as you can avoid forking the project, you can reduce your maintenance cost by leveraging community contributions and support.
However, the flip side is that you do not have full control: the community often has different needs than your own, and it's not always easy to convince the community so that the open source project caters to your needs. This is double true if you are _just_ a user.
The alternative of course is to create a fork and either take it in-house or try to get the community to rally behind your fork. In either case, you either lose the community's support entirely or reduce its effectiveness, and the same question/burden of long-term maintenance strikes you back.