I think FeverBee is one that comes to mind for professional community consultants. They had a Discourse forum – not sure what happened to it, but it was at https://experts.feverbee.com/
The professionals were not necessarily tech-oriented – they were in community management – but yes here they shared insights with each other and so on.
Yeah i get what you’re talking about – having observed for tech & game industry connections and politics connections at meetings or conferences.
For the inertia with most of these conversations between professionals in their fields happening on LinkedIn, Facebook, Twitter, and Discord (which is popular for professionals in the games industry here), etc. the main advantage of those platforms is the “network effect” a.k.a. “everyone is already on there”.
One of their advantages in this regard is that they’re centralised – whereby you make one account and have the same account for everything v.s. Discourse being decentralised (whereby your account on each instance is totally unlinked to your account on every other instance)
So when you’re sharing a business card or just your username or whatever, people usually share existing twitter, linkedin, instagram, discord accounts and so on, because it’s just a username to add as a connection on that site, rather than having to sign up to a whole new site.
I think Discourse could fit the mailing list case well because it has done that for existing mailing list communities.
But yep I get what you’re talking about - I think there’s definitely a good case to be made for trying to fill these spaces where professionals are having conversations with using a more discussion-oriented platform like Discourse. Discourse has done that for hobbyist communities, and it would be great to have more professional communities on board too.
I think the main trouble I would run into when trying to promote Discourse in the past is mainly that everyone else is on some other centralised platform – the Platform of the Day (even when it’s terrible for discussion), like first it was Facebook and now it’s Discord — and they can’t be bothered making an account on another site.
Like if i ask my friends or connections who are already on Discord to join a brand new Discord server, they will happily do that, compared to joining on a new Discourse instance (because they have to make a new account)
The question to beat is: why should I bother to make another account?
I think like what has been said often around here, “content is king” 
You would need to have quality content, resources or connections that people can’t get anywhere else. Then they will sign up for a new site.
You might want to see if you can get a few professional connections to start helping to seed content on your site, and modelling what you’d like it to be.
This can also be surprisingly a hurdle too though – like they might ask ‘why should i post there when we are already talking over here’. (And inside, I’m like ‘coz these platforms are horrible for discussion 
’)
But so what it means is that it has to be more compelling than that for them to join – it’s not just the technical benefits of the new platform, but the resources and connections that they can get there. (Which is hard when all the connections are already on another platform!)
Either way, I’m super curious now about what sort of professional non-tech community you’re hoping to start or keep developing, and I would actually like to be involved to help, if you’re up for that – regardless of the field. I’ve done community moderation (moderator & admin) on a Discourse forum, both volunteer and for work before.