Even with ZNC and persistent webchats like IRC Cloud, most people are connecting via IRC client and are doing so over WiFi and Mobile connections with notoriously hideous connection stability. Even a channel with 100, let alone 2000 users, becomes quickly overwhelmed with people unable to stay put in the channel. Or they're connecting via webchat where "we're just a browser tab" and are constantly quitting and rejoining every 10 minutes Firefox crashes.
