I would say they were granted a release from those memories as a gift, a return to the natural progression. Not many people remember their childhood friends, at least not vividly. Moreover, there is too much pain and insanity in those memories. Consider Stan Uris. He never forgot, not fully. I can prove it in the text. When he saw Bill's book he recognized Bill's name. He picked it up and read it. His wife even commented about how it gave him bad dreams. Stan, who is the one who actually made the others promise to return, knew he wouldn't (or more correctly couldn't). Stan didn't forget everything. He held on to just enough that when he got the call too much came back. It killed him.
Now consider the Losers that survived. They lost friends. They saw things beyond imagining. Would they ever really get a good night's sleep again? Could they have a normal, happy life? I think they earned the blissful peace of ignorance. The monster that haunted Derry was a stain on reality, something from outside of everything. Mike Hanlon realized that it wasn't just his memories of it that would fade. He suspected even the things he wrote down would end up fading or erased. Purged of It, reality was cleansing itself of all trace. Whether you believe this is the will of the Other, the Tower, or simply the natural healing properties of the world the end result is the same.
Well Bev and Richie have a cameo in 11/22/63.
Personally, I suggest just reading other King novels for their own merits. Don't seek them out based on a quest to find the Losers or to revisit Derry. That will happen in good time. I know it is hard to let them go after having come so far with them, but do it we must. My own personal rule is "seek not sequels lest ye find them." When you need the Losers again, reread It. The great works of fiction can be read over and over again and never lose anything for the repetition. I long, always, to rejoin...
The Losers of Derry
The Rabbits of Watership Down
The Fellowship of the Ring
And by a strange quirk of this life... I can... always.