There is some demand for those PCIe slots. Far fewer without GPUs, but there are definitely use cases where people will pay $3K for mostly IO (audio being an obvious case, network being a more niche one, unaccelerated video is one I’ve heard and don’t really understand but admittedly I’m very far from the target market). And of course the fairly obvious PCIe storage expansion use case, which I don’t think should be discounted (although it probably has been for this gen of MP).
The speculation I’ve seen (on Ars forums) about RAM is generally a consensus that this is a stopgap, with future base volume increases expected and a future multiple of die connections (a so called “Extreme” being a 4x Max). I don’t think RAM expansion is coming back to the Mac platform for the foreseeable future outside of some kind of hypothetical way to add more SoCs.