Half of the six third-party characters we currently have were added through DLC. Unless people were shoving each other out of the way to play with Pac-Man, I highly doubt any of the three base third-party characters can take all the credit for Smash 4 selling so well on launch.
If people were persuaded to buy Smash 4 before Ryu, Cloud, and Bayonetta were revealed, then I'm sure they'd still end up buying Smash Switch if no new third-party characters got added. Yes, third-party characters are very exciting, but they aren't a crutch that the series desperately needs to survive. If sales truly are effected by a lack of a certain third-party characters, I would imagine said effects would be very minimal.
I guarantee you that Sonic contributed heavily to sales, which is a big reason why we got him back. There has never been a character more wanted than him and on top of that, he's the mascot of Sega, the former rival to Mario. He's also among the most important franchise in video game history for that reason and what keeps Sega's light on the gaming business when their niche franchises cannot. Sonic is their bread and butter and the deal favors Sega and Nintendo far more than it hurts.
I see where you are coming from but consider the expectation Nintendo has to sell Smash. All five games, especially Brawl and 3DS & Wii U, are incredibly successful games that raked in hundreds of millions of dollars and each game have only gotten a bigger budget. That means the stakes are higher than the previous game and the same likely applies to Smash for Switch. Smash, the third or fourth biggest IP (after Mario, Pokémon, and maybe Zelda) is not just a huge franchise, it's a pillarstone that defines each console generation for fans. Smash even affects franchises' future - Fire Emblem being the most prominent example - and set the bar that all crossovers and fighting game series have not even gotten close to follow. When you have thirteen million dollar sales, even supposedly minor mistakes can have major consequences for the franchise and Nintendo as whole. Smash needs to not only thrive, but be the premiere crossover event of the generation and third-parties help with that. To survive would defeat the purpose of making Smash a crucial Nintendo franchise
That's why removing third-parties all together are a bad decision. Even if most of us would buy it again, a significant minority would not. Even cutting Sonic will result in a lot of missing customers because now they don't get to relieve the Mario vs. Sonic rivalry in the latest game. Now imagine no third-parties and you remove one of the biggest hype factors for the game, adding on to disgruntled customers who reasonably expect third-parties. If Nintendo are hoping Smash for Switch will sell 10+ million copies, they may not be able to achieve that without third-parties.