I was wondering if there was a metaphorical "wall," Shanks ran into as that would make a lot more sense why he's been doing fuck all. Roger's only issue was time, he didn't have enough of it.
The Yonkou themselves don't seem to be that for kind of absolute barrier for Shanks. Shanks walks on Whitebeard's ship and clashes with him and then somehow prevents Kaidou from going to Marineford. Even when we say the "yonkou are in a stalemate," we know that's a bit untrue being that Whitebeard never wanted to be Pirate King and not until Ace got on board did he even begin to have any aspirations for Pirate King [to make Ace the Pirate King].
The only two possible "walls," I can conjure up is the nonavailability of all 3 Ancient Weapons. We are aware Posiedon is only born once every couple hundreds of years and Shirahoshi wasn't alive or old enough to be a factor in Roger's and Shanks' journeys. And then for Shanks it's possible Blackbeard somewhat "changed," Shanks' goal. In the sense that stopping Blackbeard became a priority over following Roger's will. And at the time "stopping Blackbeard," would have been nigh impossible because Blackbeard was still under Whitebeard's protection at the time.