Well, I wouldn't call the plant an icon. Recognizable, sure. But not an icon.
Which is literally what Sakurai said. so it doesn't matter what our opinions are on that. He's not wrong. Since iconicness is literally based upon recognizability. PP is known to all. That makes it an icon in the end.
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As for Richter, the primary reason he's in is because he was an easy Echo. Maybe looking at the ballot helped Sakurai realize that he could please a number of people by adding him, but said ballot influence would have been insignificant if he couldn't have been an Echo. And besides, I think it's highly improbable that Richter's ballot presence was anywhere near the likes of other Echo-able characters like Dark Samus, Chrom, or Shadow (unless perhaps he was strictly looking at Japanese votes but even then I'm not so sure).
I'd say the primary reason is for the "looking at both sides of the seas to please everyone." The echo part is more of a good way to make both easier. Simon basically has a moveset because Richter existed, and vice versa. This isn't much different from Zelda/Sheik who both required each other to get in, since they were tied at the time via a unique moveset mechanic. That was Sakurai's core idea for the princess and the ninja. The fact they got split was a 3DS factor, though it did help with balance. It also helped Zelda was awful on her own, and he was well aware people played them separately too. Pokemon Trainer was done differently, forcing them as a team, so you couldn't play one over the other(and thus, was handled better overall. It still had issues, but they got the point across by actually forcing switching).
You're making a huge error in why Richter got in; he wouldn't have been chosen if Castlevania had little votes. It was the franchise as a whole that was massively popular. Richter had to be the first of the core Belmonts from the beginning to get in. Being a Japanese icon was of the utmost importance, just like Simon is the US icon. It went both ways. They were both chosen over Alucard specifically, not just one. Being an Echo is the icing on the cake and wouldn't have mattered if Sakurai didn't want to appeal to both sides of the ocean. It's the secondary concern in the end. Or to put it simply; it was Alucard or both Belmonts. Not Alucard or Simon. The extra factor was how to implement them. But since they were both added together, that would also mean that Simon's primary concern was using Richter's moveset as well, not because he appeals to the US and is chosen(with Richter) over Alucard. So either the ballot was the core reason for both, or neither. They're that tied to the hip.
Chrom, Dark Samus, and Daisy were directly voted for, but the Echo part was key. They were not really plausible without it. Hence, entirely different situation from the other Echoes. Richter is the only real stand-out Echo by being just as important as his counterpart. Ken is harder to say on the exact points. How much did the ballot influence his addition? A lot? Was it solely cause he's "the" clone? It could be both.
Though I get the ballot relates to the topic loosely, this is a pretty clear off-topic thing now. Yes, it's pretty blatantly obvious the ballot influenced tons of characters. This includes how Golden Sun got so much extra content. This affected Castelvania as a whole. It arguably helped Isabelle along with many other factors. King K. Rool and Ridley absolutely got in due to the ballot(directly said by Sakurai, along with specifically Simon and Richter's additions). Dark Samus and Chrom were outright implied due to "highly voted" without literally being said. The chances of him not meaning the ballot are small in this case. Banjo-Kazooie was probably the case too(he didn't exactly say that either, but I might've missed that one).
In the end, it's highly influential, for a good chunk of 1st and 3rd parties. If you were to include Ken as well being possibly influenced by the ballot, that means all the Echoes introduced in Ultimate had ballot reasons to be in. Which is... actually highly believable. The only one that's arguable at best is Ken due to having far more reasons than that. That, and the idea of Echoes wasn't really introduced till later, so he had less reasons to be voted on, since that was the core reason people thought he'd be in as well as wanted him in. They knew he was the perfect spot for it. Of course, I'm sure he had some votes too, but not as high as someone like Ridley either. It's also a matter of timing. The Ken "we want him" phase came in during Echos' announcement, not beforehand. Way after the ballot. It doesn't mean he had no popularity, but that's when it outright exploded. It also helps that Verge leaked a lot of information, adding to it more. I think he leaked Ken a bit later, but I forget the events.
...Ironically Verge's leaks were bad for Golden Sun(but to be fair, we jumped the gun on the "leaf" item too. The mural fake and the Rathalos picture were a different situation too. Rathalos looked like a clear reference to the game style, and the mural fake fooled a lot of people anyway. That, and Box Theory VS Mural didn't help at all).