I think the Absolutely Safe Capsule theory is the most likely. And certain other characters who would have made perfect sense as spirits, like Giygas, were also passed over. Keep in mind for example, that for some reason, almost none of the human characters from Pokemon have ever appeared as trophies or spirits, despite many of them being very popular, more popular than many of the fighters in this game. So I don't think these strange omissions necessarily mean anything.
In the case of Giygas, it sure is bizarre, but he also never appeared at all as a trophy or a sticker, contrary to Porky and his statue (who were both considered bosses in Brawl according to Sakurai).
Same with the humans from Pokemon. Except Misty and Oak in Melee (who use their anime appearance), they never were represented in any form.
Another strange absence, to an extent, could be Adeleine; but again, contrary to Porky, she never appeared or had any major role in Smash.
Sure, Porky
could be in the ASC. But that wouldn't explain why they also skipped the Porky statue, nor why they decided to make one specific joke with Porky but couldn't find the correct sprite three times in a row now (first time when they added the spirit, second time when they did a fix for many spirits including Masked Man, and finally the third time when they went ahead and actually bothered completely changing some Earthbound spirits).
If they never touched the spirits, I could understand. But they touched the ASC and Earthbound spirits twice already, without "fixing" it. As far as I'm concerned, Porky is still completely absent.
I realized that there is no way in hell Porky's "absence" points to his inclusion anymore. It's impossible that he was planned 3 years in advance with the absence. I think it's just best to give up. It's been a good ride.
We know for certain that they considered and possibly negotiated for Steve five years prior to his inclusion. Possibly the same case with the Square Enix characters to allow Cloud in. Plus even if it's not Porky, the last character would have been planned two years in advance minimum anyway (mid-late 2019 was when the plan for FP2 was finalized officially). Why three is impossible?