I think that is the reason for the misunderstanding.. FDIV was not really insidious in the way you describe. It was 100% predictable, certain bit patterns always gave the wrong answer in the quotient on the affected hardware and it had a very straightforward software fix (with a performance effect sure). You could demonstrate it immediately, but it really wasn't severe. (Q9 and Q10 http://www.trnicely.net/pentbug/pentbug.html)

Rowhammer is a much more complex errata and I don't feel qualified to comment on, especially the safety of the published mitigations, but it is in a class of bugs where the outcomes are not generally predictable due to more variables involved.

My reason for replying initially though, is that I don't think that the line for what types of hardware defects are open to software workarounds is so cut and dry, and I don't think many people outside of kernel/OS dev realize how many errata are on the chips they use everyday with workarounds they don't notice.