Why I think Roland's Quest is Eternal
I know that this topic has been debated endlessly, no pun intended, but here are my thoughts after many years of reading and rereading the series. My opinion is that Roland's quest for the Tower is eternal, and that no escape is possible for him.
The Tower created Roland through distillation via the Line of Eld and the tradition of the Gunslingers. While the Gunslingers served a purpose in the protection of the White, the true end of the Gunslingers was to create Roland. All things serve the Beams and the Tower, and so the Tower created the conditions, both good and bad, that produced Roland. Once Roland was created and perfected, the Gunslingers were no longer needed, and Gilead was allowed to pass. All of this became inevitable when the pink glass showed Roland the Tower and Roland gave up everything else for the quest. At that point, Roland's concern about Farson's men even shifts from the main concern to the fact they are now in his way on the quest to the Tower.
Just like Roland uses his hawk, David, to win his trials - so too does the Tower use Roland to achieve it's end and self-preservation. Roland's will is bound to Ka. He gives up everything to pursue the Tower, and his will cannot be separated from the Tower.
This is important because the Tower needs Roland to serve as it's ultimate protector. He is a sort of white blood cell and knight errant generated by existence itself, a recursive correction mechanism designed to attack and purify whatever threatens the Tower. Since the Tower is under perpetual threat at all times across its infinite levels, Roland's quest must also be infinite.
Furthermore, the Tower actually needs to be under threat as a requirement for its own existence. The series is, at least in part, about story, legend, and narrative. These things require conflict. They require threat and danger and monsters and villains. Threat and danger and monsters and villains require a hero, and a hero requires sacrifice. Roland is therefore both the perpetual guardian to and sacrifice for the Tower and all existence. He is the very personification of the hero cycle.
While others will join him on his quest, it is noted that they all meet a bad end. Roland will not hesitate to use them, like David, to reach his goal. He does this not because he wants to, but because he must in order to achieve his quest. But in a real sense, his companions' deaths are a mercy compared to Roland's fate. They achieve glory in pursuit of the Tower, but Roland can never reach the clearing at the end of the path and must carry on the burden of his losses for all time.
Ka is a wheel that never stops, and Roland is the knight that is bound to serve that wheel. Other heroes join that process - perhaps different ones each time - but Roland is the perpetual agent create and perfected by the Tower.