If they just do some random banner ads on the app - no one will care and it will do just fine. Advertisers will hate it and it won't bring in much revenue.
If they implement some kind of ad popup window, advertisers will love it, but people will hate it and stop using Discord.
If they do some crazy AI crap where they actually insert voice/audio ads into your audio stream - I'll be impressed at the level of intrusion. Kids may roll with it since it won't interrupt their gaming, just their chatting.
If they do as Grimlakin is suggesting and actually listen in on conversations to provide relevant ads - that will get them crazy ad revenue. I mean, Amazon does this blatantly (and people pay for the privilege of having Alexa), I suspect Apple/Google do it as well. So it isn't unheard of. I can imagine how a lot of the typical gaming banter would transfer into advertisements for butt plugs and lingerie for "your momma"
But it all depends on how the ads are implemented/presented.
I don't use Discord anyway, so whatever they do I'll just sit by with popcorn and watch.