Can someone explain why Corrin/Kamui is a bad inclusion for sm4sh?

From what I understand, there are two main arguments:

1. It’s another Fire Emblem character.

2. It’s another swordfighter.

What I really don’t understand is why these are problems.

1. This brings it to 6 Fire Emblem characters (technically 5 since Lucina is just a Marth clone). Zelda has 5 characters as well, and Pokemon has 6. The Super Mario series has 8, to be fair.

2.  I don’t get why it’s a problem to have another swordfighter. With Corrin, that brings the number of swordfighters up to 12. There are 58 characters in total. Swordfighters don’t even comprise ¼ of the roster, heck, they almost comprise 1/5 of it. Plus, Corrin is a very unique fighter, and extremely different from all the rest of the swordfighters already in the game. Most of his attacks hardly even utilize the sword.

So, what’s the issue?