Rumors of the Democratic party's death are greatly exaggerated. They still have public opinion on their side, and they still won the popular vote. If anything, it's Republicans who need a suburban strategy.
Not to mention that because the president essentially becomes the face/leader of his or her party, what people think of that person inevitably bleeds down at least somewhat to the rest of their party. It's part of why presidents cost their party downballot in midterms. Just like Obama, Trump is going to continue influencing the people he pushed away throughout his presidency. This is why it is so important to get the right person in the White House - if you put someone hated in there, they could seriously hurt your party. People tend to focus too much on that one person at the top.
http://www.vox.com/2016/9/5/12712932/american-state-government-federalismThe fact is, the average voter does not behave like Atlas posters. They don't know much about many of the other offices. If these suburbanites didn't like Trump in November and continue to not like him by 2018-2020, then I think its inevitable that Republicans in those regions suffer at some point.
Lastly, it's worth noting here that as educated Millennials
(white or non-white) grow up and spread out, they will most likely begin tilting certain suburbs more towards Democrats. The electorate is not static. It constantly changes, and when one party completely ignores and even persistently alienates an entire generation, they will pay for it sooner or later.
