We need a constitutional amendment.
If SCOTUS writes an opinion on a law like DMCA and then the legislature repeals the DMCA, it doesn’t much matter how SCOTUS interprets it because it’s not a law anymore.
Now, if SCOTUS's QI theory is based on the text of 42 U.S.C. § 1983 then certainly the legislature could change/update/replace the statute with something worded differently.
But it isn't clear that their decision is based on that (or any other) statute at all. Some interpretations of Harlow vs Fitzgerald seem to point more to the idea that SCOTUS decided that this immunity was derived from other aspects of the totality of the constitution, USC and precedent. Others see it more the way you suggest, which is that SCOTUS concluded that QI existed based on specific statutory language.
Apparently, we don't have to wait that long to find out, since they will be reconsidering it starting tomorrow.
http://www.paufler.net/brettrants/161_amendments_graphs.html
I wouldn't consider that indicative of the ease of passing an amendment. Perhaps you do. It appears to require an increasingly-hard-to-get combination of (1) widespread agreement that a problem requiring an amendment exists and (2) widespread agreement on what the nature of the amendment should be.