We can make all the academic arguments about the legality of secession, but the reality is that it's not just an academic argument. Our fiscal position as a nation is severely eroded and it's very possible that the next recession, if deep enough, might result in the Federal govt no longer able to fund all of it's obligations.
If for example tax revenues fell and expenditures were to rise at the same percentage points as the last recession, we'd be running $2T dollar deficits, this time with an ailing oil patch and a China heavily in debt and with a stalled economy. If in any way that balloons up toward $3T and interest rates spike, the interest will begin to eat up the bulk of tax revenues within a few short years if those deficits can't be brought under control.
If the Federal money fountain stops, we are going to see extreme instability and breakdown in civil order. If DC cannot step up, the states will have to. At some point, in order to keep thing from spiraling completely out of control, the states will do what they have to do, and it won't just be Texas breaking away.