There is also a third possibility, that some intelligence agency invested a ton of cash into finding abusable exploits in these systems giving them the same access a backdoor would provide.
Also from the Snowden leaks, we know that they have programs with budgets in the millions into finding similar exploits and that there were similar programs outside Snowden's clearance. And though a bug may cause the same damage to the economy, it wouldn't hurt US prestige in the same way.
Russia and China would get Windows source access for instance.
If I were a three-letter agency, I'd bribe/blackmail somebody into inserting intentionally vulnerable code. After all, sufficiently advanced malice is indistinguishable from incompetence.
We've often seen that the code inside firmware, secure environments like trustzone, etc tend to lack many of the mitigations for the classic vulnerabilities. Just rewrite one of the ASN.1 parsers in the ME (I'm sure there's at least one), "forget" a bounds check in some particularly obscure bit, and you'd have a textbook stack smash.
But really, it's not about operating systems not using RDRAND at all - it's fine to use it as one of the entropy sources; what you don't want to do is use RDRAND directly instead of CPRNG.
It is also known that they have had backdoors in commercial systems as they came off the shelf, but I think usually those were CIA owned and controlled companies like the crypto AG phones.
What is unknown (pure speculation) is whether, for example, Intel CPUs come backdoored straight from the factory floor? On the one hand, that would be a powerful capability to have, but on the other hand, the risk of exposure and subsequent damage to the US economy, prestige, etc. would be non-zero. So it's hard (for a plebian like me, anyway) to estimate how those costs/benefits might be weighed up by the US government.