[IshKebab]

Architecture tools are way more niche. I think a better place to look is mechanical CAD: how many decent open source mechanical CAD programs are there? Approximately one: SolveSpace. And that is quite basic - even simple stuff like bevels is not supported. Absolutely nobody whose livelihood depended on it would use it.

It's just too niche and too complicated for there to be enough developers willing to writing it for free.

[Ericson2314]

My uneducated guess was that engineering tools have a lot more hard requirements than architecture CAD. That's why architects higher engineers to say whether their buildings will fall down!

[rbanffy]

How much money would be needed to turn SolveSpace into something usable?

[neutronicus]

Call Siemens and ask what they charge for Parasolid. Hint: you have to ask, so...

Computational Geometry is hard. FOSS CG Kernels are not going to spring from the ether. CGAL exists, but its target audience is CG researchers. OpenCASCADE exists, but it's really a loss-leader for proprietary extensions and consulting contracts.

[rbanffy]

Maybe they can spring from a consortium of interested architects.

[IshKebab]

At a complete wild guess I would say something like $50m.

(Obviously it depends what you mean by "usable" - I use it now, so in a sense it is already usable. I guess you mean to get to the point that businesses would rely on it.)