The argument is: what else are they even attempting to offer? It would probably be too much if it was last time, because there were still characters to choose from. So much so, apparently, that Wolf had to be cut. This time, however, the only "missing" characters besides main franchise content is Ridley or King K. Rool, both of which I doubt under the premise that they did not happen last time. Ridley, was more technical. King K. Rool, or any other Kong, really, I have no idea. My theory is that Nintendo has a problem with Rare, but that is just blasphemous to suggest.
You just have to look at the intended purpose of Smash in a modern context. It is a cross-promotional machine. Why would they neglect Arms, when they have virtually nothing better to offer? And I believe they understand that, we got Inklings before any other Nintendo character this time around. To me, that shows that they understand the importance of new IP. It was almost an admittance that they were left out last time, in a way. In order to make money, you have to spend money. Really obvious stuff to a business, even one as inept as Nintendo. Furthermore, this is marketing toward Switch fans and Arms is literally one of like 4 Switch exclusive games. One of which was a "party" "game" and the other was the pseudo-port. I somehow forgot Odyssey, but that was there. Arms was at the forefront of early (post BotW) Switch marketing. If they had no faith in it, they would have "waited to see if it proves itself" back then, which they tried and failed with Wonder 101. I believe they learned their lesson.
If Arms was too late, so was Xenoblade 2, and I am sure most people doubt that notion. I understand that Xenoblade vaguely "proved" itself, but X2 had not. I would even go as far as to say that if Arms completely bombed, we might still see the character because they might want to try and salvage the IP. It did well enough, though. Arguably the best for inclusion in Smash. Splatoon did so well, that they put it off. Doing just "okay" is the BEST reason to include them and up the name recognition.