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Thanks for posting an explanation, but you seem to have missed the main point of the argument.
The thing is, no more than two of those have been on the team at the same time. I don't think you're understanding Kira's (and my) argument; it's not about the fact that Ash hasn't captured an Electric Pokemon in fifteen years, it's mainly about Pikachu. Pikachu is Ash's starter Pokemon, and will always be on his team. Yes, Ash has seven Water types, but they rarely overlap. Pikachu won't come off Ash's team, so an Electric type will always overlap. I highly doubt they'll have another Thunderbolter on Ash's team.
And that's the truth tea. When all is said and done, I doubt the writers will have Ash capture another Electric-type Pokemon since it would just overshadow Pikachu, which wouldn't be a good thing since Pikachu is, and has always been, the most marketable Pokemon in the anime.