The GOP appears to have enough votes in the Senate to appoint Trump’s nominee. If they go ahead with this before the election, Democrats have vowed to increase the number of justices on the Supreme Court. This would most likely be the start of a major Constitutional Crisis, and obviously that would not be good.
There’s a lot a stake with a conservative majority on the Supreme Court. Outside of the ACA (Obamacare), the U.S. government has failed to pass any substantive social legislation since the 1980/1990s. That means that the legality of things such as abortion, gay marriage, gender discrimination, sexuality discrimination, religious discrimination, racial discrimination, etc. all rely on decisions in Supreme Court cases. Of course those precedents can always be overturned by a new ruling, which is why this is such a big deal.