I don’t think these two things need to be bound together. People can already today depend on platform specific things just by not publishing wheels. Adding these tags naturally follows that, where people would need to manually install items from the OS before they used them. Adding some mechanism for automating this would be a good, further addition, but I think they are separately useful (and even more useful and powerful when combined).
OK, fair point. I don't disagree with any of that. As long as everyone has a clear idea that this is part of a phased approach so the wheel tags should be ultimately upwards compatible with something that does dependencies, this all sounds pretty good (and an incremental approach with benefits at each stage is more likely to succeed anyhow).