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The much more likely outcome is that people just won't bother even playing. People have their preferred pokemon and being unable to use them is quite frequently a deal breaker for them. Especially when they're limited in arbitrary ways.
I don't think that some types having more than three starters is a particularly big deal, personally. It's not really going to stop people from using those types or anything and as long as they do at least have three then there's still options.
Perhaps a good compromise would be to make sure that if a pokemon is of a given type, then they should be available for players who chose that typing. Lapras being available for both ice and water gyms, for example, or Gardevoir being available for both fairy and psychic gyms.
Did you actually read what they said? Their complaint was that pokemon that have fairy as their second type aren't considered valid by the game for a fairy team. I'm not sure why you jumped to the conclusion you did because the only thing it has in common with what Opresiet said is that both...