I think you forget just how business-friendly this country is. Nearly all states in the union are at-will jurisdictions (some with no exemptions). Of course, the basis for terminating at employee must be legal (e.g. - non-discriminatory), but I doubt termination on the basis of vaccination status will be held to be discriminatory/illegal. I'm not sure if there is any caselaw on the issue nationally, but it's quite unlikely that you'd see large corporations (e.g. - Google) issuing vaccine mandates if they felt they were not on solid legal footing to do so. Now, don't get me wrong, I'm sure there will be litigation over the issue, but I'd expect that litigation to be brought against the big players first (since they have deep pockets), and I expect those big players to put up a fight. I wouldn't be surprised to see SCOTUS take up the issue, especially if you see inconsistent decisions amongst the circuit courts.
Overall, COVID is not good for business, and the large corporations that have tremendous influence over this country recognize that. America is a ruthlessly capitalistic country, and you can bet on vaccine mandates being deemed legal if unvaccinated employees (or individuals, generally) are deemed a potential threat to profits (hint: they are, which explains the recent about-face you've seen from certain politicians and media outlets ... big money has spoken).
