The difference is one motion requires users to install and learn a completely new application that likely has a different feature set.
The other requires them to simply update their server list.
If someone has been using IRC for a decade they likely have some automated workflows built out. If they had put in that effort with slack bots or whatever - being forced to abandon or start over from scratch is a PITA.
I agree that it might be harder to switch if you have Bots or some other automation running. Although since it's just text it should work the same after changing read/write API's.
The other points seem a bit exaggerated. The "new app" to install is just a browser usually.
I used IRC a lot in the past but being a protocol also means it's barely been updated in the past. There's so much functionality in Discord / Slack and others compared to IRC that dwarf the negative aspects of "learning an entire new app" whenever you want to switch.
I'm part of a Discord community that has many, many bots handling idle tasks. We play an obscure video game, and these bots help keep track of various player's ranks, skills, which platform (Switch, PS4, PC, XBox) they're on, etc. etc.
The thing about chats-with-automation is that the automation part is strongly tied to the chat.
All these Discord bots will be a pain-in-the-ass to switch over to IRC, and vice versa.
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Even on the Freenode #C++ community, they had a FAQ-bot to spam newbies with links whenever a common question was asked (or which pastebins to use, or whatever). A huge part of IRC (and Discord) is about automation / bots, and the APIs are just not compatible.
To switch from Discord to Slack (or between any two protocols), you need to learn how to use the new app, set-up new integrations, rewrite your bots, and hope you did not depend on a feature Slack is missing.
Can people connected to Freenode join your channel on Libara? If not, what exactly is the difference?