The following is a thought experiment, so please do not take too much umbrage.
In the 2016 election, Trump [earned] 58,501,015 votes and Clinton [earned] 57,099,728 votes - if we exclude the votes in California for both candidates. Considering the matter on a state-by-state basis, it’s funny that California would negate the popular choice of the 49 other states. Consider further that even if we also exclude Texas’s votes, Trump still wins the popular vote in the remaining 48 states. That is how big a difference the people of California can make in a popular vote system.
I want a candidate to be made to appeal to as broad a swath of America as possible. I abhor that pretty much every GOP candidate in the 2016 primary bowed to the ethanol lobby in states like Iowa. And yet, it is one example of how candidates are made to appeal to interests in individual states. If you think the ethanol subsidies are great, this should appeal to you.
Voting demographics are not divided along state lines. You don’t have one state full of liberals and another full of conservatives. You just have all states with slightly different mixes of all the national voting demographics. You simply cannot target California voters as a whole for no other reason than that Dana Rohrabacher and Maxine Waters are both Californians. You can only target one voting demographic or another, and pick up ideologically-aligned supporters across the country, possibly picking up states in the process.
Yes, there are state issues like ethanol which may gain you a few more percentage points in specific states, but that’s really only a few percentage points (offset in an NPV system by equivalent losses in other states–these offsets exist in the electoral system too, but they may be in states you have written off). Also, larger states with more diversified economies tend not to have one issue that appeals to the whole state, except in that it may appeal to whatever’s the dominant demographic in that state.
So sure, California can throw the election one way or another. Or Texas, or Florida. But the margin of the last election was 2.9 million votes. That’s Kansas–consider how many Kansas Democrats don’t vote because the current system ensures their votes will never count. If nothing else, the NPV should improve opposition turnout in “safe” states on both sides.