> They can be if the policy was that individual employees needed to self enforce retention on XYZ topics and employees were provided training to that effect.
Whatever the policy is, the company is obligated to preserve evidence. Internal policy does not change that.
> The Feds can not say a company must retain all records going forward
Not only can they, that's literally what the law (federal law for sure, and separately the law of every state, to the best of my knowledge), does require regarding evidence related to ongoing and reasonably anticipated litigation. There is an unconditional requirement to preserve evidence that is known, or reasonably should be known, to be relevant.
> There is no mens rea element to having system constraints due to an existing IT infrastructure budget
The legal requirement to retain evidence once litigation begins or is reasonably forseeable is not conditioned on “so long as that is easily doable within your pre-existing IT infrastructure budget”. If you don’t have a sufficient budget to meet your obligations when you become aware of them, you are obligate to take any steps reasonably necessary to meet those obligations, and if you fail to meet the obligations, there are legal remedies within the relevant litigation for simple failure without regard to mental state (though willful failures have more significant potential remedies within the case, up to and including instantly losing the case without trial.)
Mens rea also matters a lot for whether additional civil and criminal penalties beyond sanctions in the immediate case are available (such as criminal charges of destroying records or documents to obstruct a federal investigation under 18 U.S.C. Sec. 1519), though.