The reason the dogma of Free Software exists is because often in day to day decisions it's easy not to think about the long-term effects. Right now, you have a neat chat tool. Maybe in 10 years, you have a problem where Slack has a monopoly on all project chats, and abuses that power.
Today, you can look and see "Hey, Slack is a cool company. They get it. This isn't Microsoft of the 90's. They aren't going to abuse this power, they just want to make an amazing chat tool that improves all of our lives." (This is probably true) So it seems like it's ok to use Slack. The idea is that maybe this power corrupts eventually. Or maybe Slack as an entity may not always be run by people who get it. Maybe it's broken up into parts, or becomes a hollow shell as a holding company (cough Yahoo). Without an open platform and free source code, users are unable to move away.
The dogma is a rule that brings the potential long-term consequences to you when you're deciding what to do now. It gives you a twinge "Hmm, that feels dirty because it's proprietary. I'll keep looking". That way you don't have to imagine how Slack gets from Cool Company to Abusive Monopoly, you can just make an easy choice based on how you feel.
I generally make my choices based on a balance of time/monetary cost, feature set, user experience, service integrity (e.g., SLAs for proprietary software, or difficulty of maintenance for self-hosted/OSS), and ease of migration in the event of abandonment.
The choice I ultimately make is not always, or even often, open source. There is no blanket "always choose this" guiding rule for me.
Today, you can look and see "Hey, Slack is a cool company. They get it. This isn't Microsoft of the 90's. They aren't going to abuse this power, they just want to make an amazing chat tool that improves all of our lives." (This is probably true) So it seems like it's ok to use Slack. The idea is that maybe this power corrupts eventually. Or maybe Slack as an entity may not always be run by people who get it. Maybe it's broken up into parts, or becomes a hollow shell as a holding company (cough Yahoo). Without an open platform and free source code, users are unable to move away.
The dogma is a rule that brings the potential long-term consequences to you when you're deciding what to do now. It gives you a twinge "Hmm, that feels dirty because it's proprietary. I'll keep looking". That way you don't have to imagine how Slack gets from Cool Company to Abusive Monopoly, you can just make an easy choice based on how you feel.