Opie, the "white voter pool" is still quiet substantial. You are basing your opinion on blacks, who are not reliable, and Hispanics, who are still undecided about being a block vote. You cannot rely on early polls since they are so biased toward liberals.
You may be right some day in the future, but not what we can really see. As Dave pointed out more Hispanics in California gives you no more votes - just a bigger margin in the state. You are going to have to rely on which states have the growth - plural. Growth in particular states or districts could steal a house seat from another state, but that is going to be very minute.
Instead of relying on the future changes that you predict, it is easier to get votes by strengthening the appeal of the party to get a larger % involved in the process. Do you think it is possible for a meaningful number to switch registration to Republican after the population compares what the conservative candidate has done for the country after 8 years of nothingburger? Didn't think I would ever find a place to use that word.
Trump won and gets to stay in office so every day his backers get to feel good every day because he won but the thing is, there are no elections for a bunch of days and nothing changes in terms of officeholders and then there's an election day and things can change. It's not like we're having election days every day and Trump keeps winning.
If things keep going how they are then come election day in 2020 the Dems are going to be VERY energized. The GOP won't be quite as energized, since complacency is normal. Trump's approval rating is low even though the economy is doing very well, which ought to worry Trump backers since those ratings will probably get worse if the economy slows down a bit, which is likely to happen eventually.
And on top of that I don't think people realize that Trump's victory really was narrow in 2016. When he was bragging about how much he won by after the election, he was slinging BS. It was very close. And on top of that the Dems are likely to present a better, and possibly much better, candidate in 2020.
Just because you get to say every day "Yay, our side is in office" doesn't mean things are going well for you in terms of the next election. As far as the next election goes, if there's no 3rd party candidate (which is a big unknown at this point) I would say the Dems chances look pretty good right now. (Then again, I was wrong about the 2016 election.)