I can share you my experience, my discord server is way busier than my forum! Probably the best thing that happened to my community besides me

140 users are usually online with maybe 15 people actively conversing at any time. I have the widget on my site and also am a
Discord Partner You have to have a very large community, Twitch or Youtube channel to get accepted but you get some neat perks like Vanity url, free Nitro, free VIP server and access to the Partner channel which is pretty dope.
The Discord channel definitely hurt my forum but has made the community much better, bullshitting with everyone in discord is a blast. People get involved way more in discussions on discord than they do on the forum.
Also I sell ads on my site and also "advertise" on my discord so that kinda displaces the lost traffic on the site in the eyes of potential advertisers.