I contend (and this is consistent with my other writings on blockspace) that blockspace is an ephemeral good. The diamond analogy is flawed, because if Polkadot sold parachain slots in perpetuity it would be more akin to DeBeers selling x% of its production in perpetuity - not an unreasonable offering altogether, but not what Polkadot offers. In any case, I don’t view this point as material to the argument.
I think the rest has been adequately covered - while it’s certainly possible for more relay chains or more blockchains to be spun up and thereby create ~unbounded amounts of blockspace, this blockspace would be of lower quality due to the real security and quality properties required to generate it. Polkadot generates as much blockspace as it can at an appropriate security level for interoperating applications. A company selling shiny rocks is not really a competitor to DeBeers for a good reason.