People are used to paying nominal transaction fees for commerce transactions. Pro versions of group facilitation, messaging, and ticketing is also a thing.

The problem is just cost. These post IPO companies grew massively with sales and support teams. It's very likely much easier to get to scale nowadays. Remove the sales teams and find ways to grow on the cheap. "can't make money" isn't right, it's managing costs to turn a profit inline with the current state of things.

Once people have formed their community, they can just split any costs in-person and not go through your platform.

I'm not paying $20 in yearly "dues" to maintain a friend group that could be maintained for $0 as a group on any social media messenger.

It's just not enough money to sustain a VC-backed model or a large sales/support staff.

If OP is careful about managing expenses and doesn't set outlandish expectations for profit (or raise VC), a profitable niche is doable.

Well, hopefully, you're wrong!

The gist of my plan is to focus on a broader range of groups/communities, with the larger groups who want/need premium organiser features (or paid promotions in listings, etc) footing the bill for smaller, free groups.

Smaller communities/groups that don't collect fees from their members are likely what make up the bulk of groups that exist - so I don't think it makes sense to exclude them by not having a free option. They're what will bring more value to the discovery side of the platform, IMO.