Like I said it's a tradeoff.

For you, it sounds like it hurts more than it helps, but for others the options may be:

1. Don't use slack at all because they can't figure out how to install the correct version of libthingy.dll. Or they don't get the app at all because it's not for their platform.

2. Use slack

And in that scenario, the option they went with wins hands down.

I'm not saying it's perfect, and I still believe that giving both options would be a benefit for most, but the fact that it is done this way wasn't a "mistake", it was a tradeoff for the targeted audience solving problems that many had.