PS5 is down double digit percentage YoY and is now tracking behind PS4 worldwide. A double digit dip is the definition of plummeting. Xbox though, plummeting is being nice, that system is straight up cratering.
With that said, people thinking GTA6 is going to save the console market are in for a rude awakening. There will be a bump, sure, but no one is having record setting quarters. There's already over 65 million PS5s out there, the market has already begun to capture a lot to the casual user base five years into the gen. There isn't tens of millions of people still stuck on PS4 and XB1 waiting for GTA6 to buy the next console.