The EC did exactly what us was supposed to do this year. You cannot just win mostly coastal and high population density states and call it a win. Without the Rust Belt, the Democrats become a regional party. It is their job to have a more inclusive appeal. They failed miserably, because they did not want to listen. The EC punished them for living in an ivory tower.
The form of government in the US is a federal republic, not a democracy. Liberty is meant to be and end not a means. The EC is part of a checks and balances system meant to keep impulse decisions at bay. Trump's election was a result of the careless actions of the Democratic Party overplaying it's hand. Their impulse decisions ended their control.
I find what you are saying problematic on three counts.
Firstly, the Electoral College does not prevent regionalism, it encourages it. pikachu brings up an excellent point when he says that being punished for appealing to a small audience goes both ways. Just like Democrats can be said to have been punished for winning only "mostly coastal and high population density states", the Republicans were in the same way rewarded for ignoring them.
Think of it this way, neither party really needs inclusive appeal because there are so many "safe states" that they only need to appeal to swing state voters. For example, as a Republican, you can completely ignore Californians because even a 20 point swing will still not be enough for you to win the state. As a Democrat, there is no reason to try to broaden your base and appeal to even more of California's voters because you're going to win the state anyway, so why bother?
To add to that first point before we go on, there is a reason that the parties sometimes shift their platforms. If a national vote system was implemented and Republicans found themselves losing election after election, they would change their own platform to appeal to a larger audience. In that same manner, if Democrats found themselves in a losing position, they would start trying to appeal to more rural voters.
Secondly, the Electoral College does nothing to promote a "more inclusive appeal". Briefly, because this is not the main point I want to make, take a look over at Trump's support among America's minority populations. I would not say that he was able to garner very broad appeal with that group. In fact, both sides vie more for winning their key constituencies by larger and larger margins rather than becoming more diverse in their appeals. But back to the point that Democrats lost because they failed to appeal to Rust Belt voters. Michigan, key among those rust belt states, currently stands at 47.6 to 47.3, a mere 0.3% difference. To say that Clinton failed to appeal to Michigan's voters is a misnomer. It
is true that she didn't appeal to them quite as much as Trump, but she still received roughly half of their support.
In fact, if your point is that inclusive appeal is important, then the Electoral College is nothing but a hindrance. For instance, if your goal was to push both sides towards appealing to white men, the electoral college would make it so that the only appeals you would make would be to white men in swing states. Even though the policies put forward by both sides do tend to stretch beyond state boundaries, that is only a somewhat happy accident. Just like with my first point, the inclusive appeal that you are hoping for only has to be inclusive of people whose votes will matter on election night.
Finally, you say that the Electoral College is an important part of our checks and balances system. In a country where Electoral College voters are picked by the parties and face fines or prison time for voting against winner of the popular vote in their state, the Electoral College is nothing more than a obstacle to the presidency. I know some Democrats have a bad habit of saying Trump is Hitler, but if Hitler ran, there would be nothing in the Electoral College set up to stop him. The checks and balances on the office of president are the other branches and laws, not the election system itself.
So basically, the Electoral College is anywhere from an unnecessary extra step to a complete detriment depending on how you look at it. The only positives that it presents are things that would be even better without it.
