"This character is too dead, not relevant, and obscure to be playable. It wouldn't increase sales as much as X character. So they shouldn't be in".
-----------------------
It is single handedly the worst criticism ever because now you're treating Smash Bros as a billboard as opposed to a celebration of Nintendo history.
Isaac is currently my most wanted and it is often thrown at him. I tend to favor characters from more under represented / obscure Nintendo series than more popular 3rd parties or characters from series with a lot of representation because as a kid the most common console war insult thrown at Nintendo was that all it had was Mario, Pokemon, and Zelda. This is untrue because Nintendo probably has the most IPs out of any gaming company.
The argument is pretty hypocrtical and not very selfaware. Smash Bros simply would not be the same without characters from "dead" or "unpopular" franchises like Captain Falcon, ROB, Mr. Game and Watch, Duck Hunt, Little Mac, Ness, and more. Not only that, but including lesser known characters in Smash immortalizes them or increases the sales of their games, heck it helps them get new games. Look at Fire Emblem and Kid Icarus Uprising. Heck, poor Snake will probably get this argument thrown at him in the next decade or two.
These accusations of relevancy, simply don't hold up in 2018. We are dealing with a new landscape. Due to the power of social media and the evolution of marketing, dead franchises like Shenmue (last game was 2001), Streets of Rage (last game was 1994), Samurai Shodown, Windjammers, Metroid, Soul Calibur, Megaman, and Devil May Cry are given another chance. With the power of Smash, a lot of Nintendo's dead family members can be given another chance.
The relevancy argument is a completely darwinistic way of looking not just at newcomers, but at games and entertainment as well. This argument isn't just an attack on F-Zero or Golden Sun, but an attack on those games I previously mentioned. Ever watch a movie or show that never received a sequel because it didn't make enough money? Man, I remember when Odd Future won a music show and all the people who were mad because they never heard of Odd Future. "W-what how can something not as popular win!?" Ever see your favorite restaurant shutdown while McDonalds still sticks around? Its the same thing.
The worst part of this is that these relevancy arguments aren't coming from CEOs working at Nintendo or whatever. Instead they're coming from this vocal minority in the Smash fanbase that has developed some strange contrarian hate for the lesser known. Mario, Sonic, Pokemon, Zelda, Pac-Man, Animal Crossing, and more already have heavy content in this game. Its a signal its going to sell ALOT already. How is this hard to understand?
Ironically enough, this argument has given me empathy for a lot of contenders: From popular choices like Skull Kid, Geno, Banjo, Rhythm Heaven representation to smaller fanbases like Andy, Ray, and Jill etc. The relevancy argument is thrown around all the time to these people. We should be kind to each other, because these people throw this stuff at all of us. All of us should stick together.
Luckily, with all of these sequels of games that haven't had sequels for DECADES happening, these discount business managers, dollarstore CEOs, knockoff brand salesmen, etc. are going to get BTFO.
In this way, a lesser known or less relevant character getting into Smash isn't just a victory for that character or their series, its a victory for any dead franchise. If someone who isn't "relevant" or "popular" gets into Smash now, like Geno or whoever, its part of this awesome trend, of dead franchises and characters getting a second chance.