no formal support
Do a case study comparison between finding a problem and making a change in the specific open source component you want to use vs. finding a problem and making a change in .Net.
Oh, wait. You can't easily make changes to .Net, can you?
lack of trust in something perceived as "just downloaded from the internet"
Correct. Wise. Hence testing.
You test your own software right? Test the software you downloaded. It's not hard.
For the specific component you want to use, download it and run the test suite.
How can it be professional if it's not supported, we don't pay for it etc etc
Again. Case study comparison between a specific package you specifically want to use and your own in-house software.
If you actually download something, and actually read it, and actually compare with in-house "best" practices, you'll often have a compelling case study that shows
Open Source is really good.
Your in-house best practices need improvement.
The point is not to generalize but to download a specific package and explain it's specific merits.
One package at a time.