A year after this letter appeared in Captain America 206, the Falcon was gone. Jack Kirby's last issue as writer/artist was 214. The Falcon was written out of the main storyline in issue 217 by Roy Thomas. He made a cameo in 218 by Don Glut. He appeared in a filler short story in the back of 220. In issue 223 (written by Steve Gerber) the Falcon was dropped from the title of the series without explanation. I was reading the book at the time as a kid, and I was not a happy camper.
Who made the call and why? I don't know. There may have been a hint in the letters page of issue 222:
Steve [Gerber] feels that most of the supporting players in earlier issues of CA&F-- Sharon Carter, the Agents of Shield, etc.-- have about had their day. He would like to endow the magazine with a new cast of characters, a different type of plotline, and, generally, a whole new look to see it gracefully into the 1980s. Do you agree?
The next issue the Falcon was gone, though Gerber never did get around to giving the series a whole new cast of characters. Roger McKenzie took over the series with issue 227, as the merry-go-round of post-Kirby writers continued.
The usefulness of a sidekick for Captain America was something his original creators recognized from the start. On the very first drawing of Captain America by Joe Simon, he wrote a note saying "I think he should have a kid buddy or he'll be talking to himself all the time." Thus was born Bucky, a standard 40s Robin-knockoff kid sidekick named after the star of Joe Simon's high school basketball team.
Later writer Stan Lee hated kid sidekicks though. He killed off Bucky in issue 66 in 1948, replacing him with a new hero named Golden Girl. Bucky's death was ignored when Cap's series was resurrected in the 50s, but Stan killed Bucky off again in Avengers #4 in 1964. After experimenting with teaming up Cap with Rick Jones (the Marvel universe's all purpose sidekick, who also teamed with the Hulk, Captain Marvel and ROM: Spaceknight), Stan created the Falcon, and eventually included him in the title of Cap's new series. The Falcon did a lot to ground the series in present day New York City, and provided Cap with a capable partner who had reason to be a bit more skeptical of America than Steve Rogers. I sorely missed the Falcon's presence in the book when he was dropped.