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As you've described, the problem with any single chassis, the chassis itself can become a single point of failure even when loaded with redundant components. Independent chassis (e.g. VSS, StackWise) provide chassis redundancy although it does raise a question on software redundancy, i.e. multiple chassis are running some form of tightly coupled chassis OS vs. standalone chassis. Standalone chassis should be, in theory, the most redundant but they too create issues such as load sharing and asymmetrical path issues (e.g. unicast flooding). If you understand and trust a multi-chassis OS, I personally think it's often the "better" redundancy option.
I don't know if VSS on the 4500s support it, but 6500 VSS supports quad supervisiors, so if you lose a single sup on those you don't lose one of your VSS chassis pair.
Regarding your later post on using a 4500-X VSS pair, yes that might be an idea core for high density 10g. For twenty 10g ports each, you would need either the 16 port with the optional 8 port module or the 32 port model.
Depending on your remote rack setups, you might also consider 3750-Xs, stacked, in lieu of multiple (individual cabinet) ToR 2960S pairs. StackWisePlus is a much better stacking technology then FlexStack. Yes, the 3750-Xs are more expensive, but you might need less if you can bring multiple racks to the same 3750-X stack. (Depending on how many downstream stacks you actually need, you might also reduce your need for 10g ports on the core.) Depending on your feature needs, you might even be able to use the LAN Base models which in the later IOSs also support StackPower.
10g is nice, but it's also expensive. When working with switches within 100m, don't overlook the possibilities of gig copper Etherchannels. For example compare total cost of 8xgig (copper) build-in ports vs. single 10g (fiber) especially if special module is required.
