As far as I’m concerned, the optimal solution seems to be moving to a chat platform that has backwards-compatibility with existing IRC users whilst also making things significantly easier for new non-IRC users.
Gitter fulfilled these criteria rather well; I can connect to it with my IRC client, but regular users have a glossy web client to use instead. Discord is okay here; I can connect to it with bitlbee-discord, which works pretty well but is, as previously discussed, not something sanctioned by Discord staff. The question is: if Discord were to suddenly start cracking down on custom client users, as their ToS allows them to do, would the Rust team migrate away? If the answer is yes, I have no qualms with Discord.
Perhaps a better solution would be to set up something like discord-irc between the moznet IRC channels and the Discord ones, which both reduces setup time/faff for IRC users and moves us onto an officially-sanctioned way of using Discord.
I think @aturon is pretty correct with this analysis - if all the discussion for Rust ends up being on Discord, most people will just use Discord. However, there are still some problems with this arrangement: blind users et al. are excluded, due to accessibility issues, and, perhaps more pressingly, the degree to which users idle on Discord would probably be reduced. IRC clients use very little CPU and RAM compared to the bloated Discord web client; therefore, some people will be unwilling to run Discord in the background all the time for notifications, while IRC remains usable for them.
Essentially, Discord seems to sacrifice accessibility for some users (blind, having low-power computers, etc) whilst giving it to another set of users (new community members who aren’t used to IRC et al.).