Most people do MRs for EQMs. It's very rare to see a MR fare posted where the RDMs alone make it worthwhile, unless it's a mistake fare or there is travel hacking / fuel dumping or something else going on.

I do not believe that this is true. Common sense says that if AA is routinely giving its elites freebies whose market value exceeds the fare paid by the passenger, AA would be bankrupt. (Oh, wait, AA is bankrupt...) But I really would not worry about the possibility of free upgrades from an airline suddenly becoming taxable income--although you are free to worry if you want to. It is also possible that Congress could raise the tax rate to 99% next year, but it's really unlikely, so I don't spend time worrying about what I will do if that happens.