The first reason was that a third party candidate wouldn't have the party money machine behind them. Sanders and Trump both showed that you can raise a lot of money outside of the two parties since neither got much money from traditional party sources. An independent would get outspent but these candidates have shown that wouldn't doom them. Especially if Trump put more of his own money in.
The second reason was that you need the party apparatus for GOTV, but neither candidate had that in the primary and Trump isn't really using it in the general election. It's a drawback but might not be fatal.
The third reason was that you simply can't convince enough Republicans or Democrats not to vote for someone in the party. That might not be an issue either. Enough people on the right don't like the Republican party enough that they'd have no problem voting for an independent. I'm not sure if enough Democrats would. Trump got a good share of his support from self-identified Republicans, but Sanders got more from independents. A good share of his support was from people to the left of the Democratic party. Would a centrist candidate be able to convince traditional Democrats to abandon their own party?
All of those reasons might still apply but now we can see that they might not. The candidate would have to be high profile like Trump or Sanders, although it could be argued Sanders wasn't high profile until after he ran. You'd also need enough people in both parties unhappy with their nominee. That doesn't usually happen, but Sanders and Trump weren't necessarily popular because others were unpopular. They were popular because of who they were, their message, and because the environment was ripe for it. There's no reason to think people are going to fall in love with the establishment any time soon.