That's true that many of the things I mentioned (not all) are possible through clients building in the functionality on their end, but that leads to another pro

That's true that many of the things I mentioned (not all) are possible through clients building in the functionality on their end, but that leads to another problem that slack can avoid, which is a unified interface for all users. While file transfers may be made easier on one IRC client that some users are using, it has to be completely compatible with the user who is connected through Irssi or another less capable client. This limits the possibilities of a client (which, mind you, is actually a big selling point for a lot of IRC users including myself). And I agreed on the fact that it's closed source and that can be an issue. But like I said above, that means that there is room for FOSS solutions, not that IRC is the answer.