Everyone who plays Civ VI more seriously uses this "exploit". It's literally the first thing that comes to mind to efficiency-minded players when presented with the system, and I guarantee that Firaxis's testers do it themselves and that it's the intended way to play for more skilled players. Same with researching 60% of techs and civics you don't have the eureka for yet but plan to get, and then switching so as to not waste the eureka. It's just basic application of the system that is in place. I would never keep the upgrade policy cards for more than 1 turn when it's clearly a waste to do so. If the devs didn't want people to play this way, they would change the rules. And maybe that's what they should do.