If every trainer started off with the same Pokémon it'd be chaos! With the huge amount of trainers starting their journey every day and the amount of starters that are needed as a result, that Pokémon's numbers in the wild would decrease substantially with the number being taken increasing exponentially every year with population growth, excess of interest in training Pokémon, etc, resulting in minimal numbers remaining. This would cause the species to become extinct in the wild with no chance of finding mates to breed with, thus resulting in any predators of that Pokémon starving to death and also becoming extinct. The prey that was before hunted by said starter Pokémon would now be present in enormous quantities, thus depleting all of their own resources from the over-grown population and extincting them too. Essentially what we have here is whole ecosystem collapse which would spread throughout the Pokémon world, thus eradicating any Pokémon that you have ever wanted a chance of catching as they'll all be dead from starvation. If you want to cause the end of the world then only give trainers one starter choice - I hope you people are proud of yourselves.
Lol idk it'd be boring using the same one over and over give me lots of cute things to choose from!