The thing is, after adding Ice Climbers, Sakurai still went on to add Pit and Duck Hunt, more characters that came straight from the NES era just like them. Whether his intention was to have them represent the NES era or not is debatable but the fact that he added them means that it's definitely not out of the possibility that he could add more of those characters again.
Pit was added because Sakurai wanted to envision how Pit would look if, like the Legend of Zelda, the Kid Icarus series evolved over time to reflect our modern design sensibilities. He thought it was a neat idea to redesign the character, and apparently was charmed enough with the Kid Icarus series that he wanted to see if he could revive it by putting Pit in Smash and doing a revival game.
In other words, Pit was not chosen to
represent the NES era at all. Sakurai went
out of his way to make Pit
less of an NES character. He totally redesigned him, and didn't even really try to keep Pit's gameplay in Brawl referential to the original Kid Icarus, outside of the animation for using the hammer. He was explicitly going out of his way to revive the series; to make Kid Icarus a modern franchise, rather than a retro one, was his entire goal.
I do not think it is possible to argue that Sakurai intended to use Pit to represent the NES era (as was explicitly the stated goal of the Ice Climbers' inclusion) when considering both his actions and his actual statements on Pit's inclusion.
As for Duck Hunt, they were explicitly described as the "surprise" character of Smash 4 in Sakurai's project pitch slides. He clarified that he considered Duck Hunt to be in the same grouping as Mr. Game and Watch and R.O.B. While Duck Hunt is a one-off NES game, and I understand why people interpret them as a retro character, I feel like people misunderstand the significance of Duck Hunt's inclusion on some level. Duck Hunt's series in Smash is sometimes actually said to be the Light Gun series, which I find extremely significant.
Nintendo has a very, very long history with light gun products. They released their
first line of Light Gun toys (Kôsenjû SP) for the home in 1970, with several other products releasing through the decade, including the first
Duck Hunt in 1976, an electromechanical toy predating the NES game by 8 years. In 1973, they used similar technology for a light gun game known as the
Laser Clay Shooting System. Nintendo rented out a bunch of bowling alleys (which had been deserted because of a recession) and turned them into a sort of arcade where players would spend some money to shoot at clay pigeons (projected on a screen) with a rifle that featured a mechanism that determined whether the shot "hit" the light. They also developed a
Wild Gunman arcade game in 1974 that plays very much like the NES counterpart. (As an aside, Nintendo also produced a Wild Gunman toy in 1972 with no electronic components. This makes it the oldest series represented in Smash Bros. to my knowledge.)
All of these products, as well as the NES games, are what Duck Hunt references in its moveset. That's why they have the Wild Gunman and Clay Pigeon special moves. Because the NES games were actually the last in a long family of games that helped saved Nintendo from bankruptcy in the 70s.
Perhaps even more interesting than that is Nintendo's involvement with Magnavox. The Magnavox Odyssey, the first home video game console, released in the United States in 1972 and has a very interesting, albeit poorly documented,
history with Nintendo. See, Magnavox decided that it wanted to bundle a Light Gun with the Magnavox Odyssey, but didn't have anyone to partner with to make the guns. Fortunately, when they looked around, they found a Japanese company that was currently manufacturing light gun products. Magnavox contacted Nintendo and Nintendo agreed to produce the Light Gun for the system. It looks nearly identical to the Kôsenjû SP rifle externally, except that it includes a cord that connects it to the console. On the inside however, it totally reversed the mechanism for detecting light. Instead of shooting light out as a laser, the screen went blank except for a white screen, which could be detected by a sensor in the gun. This is how the NES Zapper works, as well. Records are spotty, but it seems like the Magnavox Odyssey was released without any modifications in Japan, as they used the same NTSC formatting for their TVs at the time that the North American market did. This may or may not have been in 1975, it's not terribly clear.
But, regardless, Nintendo shortly thereafter began working on their own home video game consoles, the Color TV Game series, and eventually went on to release their own home console which was bundled with a light gun. Their early work with the first home video game console producer absolutely influenced Nintendo getting into the home video game market at all. That history is, I'm sure, why Duck Hunt is in Smash Bros.
After all, I only fell down this rabbit hole in the first place because one of the in-game tips mentions the first Duck Hunt, which released in 1976.
In-Game Tip said:
Duck Hunt's Origins
Duck Hunt was released on the NES in North America in 1985, but did you know Nintendo released another toy in Japan with the same title in the '70s?
apologies for the essay, I just think this is too cool