How about a compromise and say that San Andreas replaces California but that San Andreas takes the land that California as a state occupies, and Los Santos aka Los Angeles is situated on an island off the coast of California?
I understand the previous point that it would not make sense to have the US' second largest city on a separate island, but that is trivial as obviously Los Santos in this universe is not the size of Los Angeles in any way. Having it like this would mean we don't just ignore that we are obviously roleplaying on an island and any characters "visiting" San Fierro or Las Venturas would hop on an imaginary flight or boat to the mainland.
See the image in the spoiler below to see a rough idea of how this would operate. Obviously, the image is comparing the sizes of LS and LA, but for roleplay purposes imagine that LA was not there, or possibly even roleplay that Las Venturas and San Fierro as they exist in GTA SA occupied that space. Like this, it would only be a short flight to get from Los Santos international to the mainland.
I'm not too bothered by either prospect in all honesty as it makes little difference to the actual roleplay, but a line in the sand should be drawn so everyone knows where they stand.