Why though? If you’re good enough to attack tank, you’re good enough to defend tank.
Then you lose that game as dps, your dps SR goes down and you decide to queue for tank/support for the rest of the day because your aim is off.
People who genuinely switch to help the team are few and far between, and have been hampered a little by this change. The reality though, is that the majority of people switch off tank/support and try to brute force a win on DPS, leaving their team at a disadvantage -1 tank or support, and guaranteeing the loss they are trying to avoid.
The change will take a while to get used to, but the quality of games is already noticeably better, and will only continue to improve as the ranks settle down a bit.