The idea that 1994 and Prop. 187 are the start of Republican problems is a fallacy that's so often repeated that people believe it's true. That ignores that from 1974 to 1992 Democrats won 27 of 37 statewide races, with 6 of the 10 Republican wins by 2 candidates. Prop. 187, occurring after the GOP slide, won by 59%-41%. So it's hard to believe that this proposition is the source of Republican problems in the state. There are a number of reasons that the Republican party started a California slide in the 1970s, but probably the biggest reason was that the Democratic party had a registration advantage of over 22% in the 1970s, higher than today's 15%. That Republicans ever won elections in California is the anomaly, not that they stopped winning them.
Likewise passing immigration bills might help the Republican party, but it's certainly not a cure-all with Latinos or other elections and it won't make a difference here in California.