No, Trump probably isn't going to die of COVID. Even if his condition becomes critical, he'll have the best healthcare possible and the odds of him being denied a ventilator are 0%. Same goes for Melania. White House doctors are going to do whatever it takes to keep the President and First Lady alive, regardless of how many resources and how much manpower it takes. But most likely, things won't even get to that point.
This still hurts Trump's odds of winning though. Maybe not by much, but a little. He's almost certainly going to miss the second debate, he might have to miss the third too. And if he does end up getting even moderately sick from this virus, that's going to make him look like even more of an idiot for downplaying it so much. And on a more basic level, it's going to make him look weak and leave people with the impression that he might not be physically healthy enough for another 4 years in office.
I've also heard some people speculate that Trump faked it, either 1. as a way to make people sympathize with him more in general, 2. as an excuse to deliberately avoid the next two debates, or 3. as part of some weird scheme where he'll take hydroxychloroquine or some other bullshit "cure" that he endorsed, miraculously have a full recovery in just a few days, and use the whole incident as proof that he was right all along. Or, alternatively, just get better on his own to show how "tough" he is, or to prove that he was right about COVID not being a big deal.
Another possibility is that he's looking for an excuse to have Pence replace him as President before the election, so Pence can pardon him for all his crimes before Biden gets into office. Which honestly might be the most credible motivation I've heard yet.
I don't think this conspiracy theory is likely - I'd say there's a 99% chance that Trump and his wife really do have the virus - but if it did turn out to be true, I wouldn't be surprised. And the fact that I can't put a stunt like that past him really says a lot on its own.