Charge by workspace(a workspace license, unlimited users) OR charge ONCE per user unlimited workspaces(maybe charge an additional fee to allow this on a per user basis at the very least). Trello's adoption will grow more than enough to make up for this absurd structure. I'm open to anything that is sane and clearly explainable to upper management.
I've been on two teams that have cancelled anything above free, or abandoned Trello because of the licensing structure. We are an enormous international non-tech company. It's not because the cost is the issue.
Think about this dialog:
Management approving invoices: "I thought we just paid for your Trello license, like two times already?"
Me: "Trello charges per user per month annualized per workspace"
Upper management: "Use MS Planner"
Me: "bbuttt.., I like this product from a company that has such a license structure, it's miles beyond planner"
Upper management: "You're an idiot. We already pay for planner and anyone in the company can use it with any team they want, use it"
Me: ". .... .. ...."
Trello is leaving money on the table, plain an simple. I could probably get more $ approved than we would pay with this license structure, but because it only provides scalability in a non-linear manner. A lower priority group with more members cost more than a higher priority group with less members... It's not about individuals, I'm part of several groups, most people using a tool like this are. Some of those groups may take an hour or two a month. Some of those groups may take %80 of my time.
And that's you're lucky enough to have management that is will listen enough to understand the licensing and think about scalability before saying "no."
What's the future of Trello? I don't know, I'm not focused on innovating in this space. I can say that if things basic as status (a "custom" field...) is locked behind this asinine price structure, it's no wonder that I haven't heard nearly as much about Trello in the past 4-5 years. I'd imaging your adoption rate stalled right in line with that.
Sorry for the rant, but this is quite short sighted and shows the lack of understanding in corporate structures. Which is a real shame for such a useful tool.