"First, not all buyers care whether museums are some day going to be willing to accept donations of their artifacts. They are happy enough to acquire for themselves such beautiful objects, and perhaps eventually even display them in private museums; or they anticipate that eventually some solution to the problem of so-called "orphan" antiquities will be found and the very caring foster-parents who purchased these "orphans" will then be permitted to donate them.
"Second, the risk of having a repatriation claim brought is a calculated one for any buyer, and depends on several factors that may reduce it substantially: where the object's country of origin is difficult to establish that risk drops substantially, for instance, and the resources available to the country of origin are likely to be scarce, requiring them to focus on the highest-end objects and on repatriating items owned by countries, museums, or universities where leverage can be exerted in the form of threats to ban archaeological digs or exchanges."
I suspect that Rothfield is perfectly correct. We should also bear in mind that some buyers come from countries which are less concerned about discouraging looting than the US or others.
I have always considered that strictly adhering to a pre-1970s provenance is unrealistic for "minor" antiquities (reasons discussed here). But we do need to establish a system that will allow collectors to clearly distinguish between artefacts that have been circulating for years and those that have been freshly dug up. Merely ignoring the pre-1970s provenance requirement is not good enough; there needs to be a pragmatic compromise rather than nothing at all.