I think anybody who doesn't want Goku in Smash would have to explain why Cloud being in Smash was OK. Cloud is about as far from a Nintendo mascot that you can get. At least Goku has been in some DBZ games that made it to Nintendo consoles.
OK I will take you up on that.
1-Cloud, unlike Son Goku, originated in a video game
2-Final Fantasy started on the NES and has had multiple spin off on Nintendo systems
3-While Cloud original game never made it to a Nintendo system, he as been on a Nintendo console before, heck in the direct where he was announced for Smash, he had already appeared in an earlier segment regarding a FF spin off where he was playable
4- Not really involving Cloud, but if we start getting non video game characters in Smash because they have been in multiple games then why not add CR7,Brock Lesnar,Shaq, etc. they are just like Mr. Goku, heck I would argue some of them would be more deserving of a spot over him if we go by recognition,sales of the games they are in.