That said, somewhat orthogonal to your point -- the amount of capital applied to create these gigantic platforms is not indicative of what people can and do create with internalized cost -- the F/OSS movement has proven that. People have created immensely valuable software, and internalized/shared the burden of creating that software. I also think that it is entities with large capital that strive (and almost always end up creating) gigantic platforms -- it justifies their heft/consulting teams, etc. motivated single contributors/small groups never set out to giant platforms, they usually build things that do one thing well (see unix) and compose (see unix) to do greater things.
More towards your point directly, you're right, a lot of things would stop making sense in a perfectly competitive market in the theoretical economics sense. Outside the fact that the model is wrong/incomplete (as all non-perfect knowledge models have to be), I do think this would lead to a certain number of competitors, but likely more than 0. My instinct is that the number of firms selling the product would be equal to the aggregated demand / costs to run a business selling the product -- and that ratio is probably greater than 1. Purely a spherical cow scenario but at least that's where I think it would go.