Out of universe:
As I remember, Trip was killed right before the founding ceremony of the United Federation of Planets, and that was more than five years after the previous episode.
If the creators of Enterprise were absolutely certain that the series would never be revived, they could have killed off all the characters just to shock the fans. So fans can be grateful they didn't.
And if the creators of Enterprise weren't certain whether the series would ever be revived, they would figure that if it was, the new episodes could happen in the half dozen or so years between the last episode where Trip or any other killed of character was alive and "These are the Voyages" where Trip or any other killed of character was killed. So they could have killed off any combination of characters, secure in the knowledge that if the series was revived, there were still several fictional years left for stories where those characters can still be alive.
And in the unexpected case of Enterprise being revived for a fifth season, the fans might find it a little creepy knowing that one or more of the protagonists would be killed in the future, but I guess that the fans could get over it. I remember reading and enjoying a book as a teenager despite knowing that the protagonist would be killed after that book and before the next book in the series.
And to fans who are saddened by the death of Trip or of another fictional character:
I say that almost every single work of fiction must happen in an alternate universe to ours. Thus reading or watching fiction more or less implies a temporary belief in alternate universes in order to suspend disbelief.
And if you must more or less believe in alternate universes in order to suspend disbelief in a work of fiction, you can believe in not merely two alternate universes - ours and the one that the story is in - but countless millions and billions of alternate universes branching off from our universe - and from the universe of the story - every second.
So the beginning of the fictional story has countless alternate universes with different events branching off from it. Each episode in a series has many different possible endings, and all of them should be equally real in different alternate universes.
So each and every one of the Enterprise characters has been killed many times in various alternate universes, and has survived many dangers many times in various alternate universes.
The creators of Enterprise could only show a tiny fraction of the millions of adventures which the crew experienced in different alternate universes, selecting which ones to make episodes about according to their various criteria.
And so "These Are the Voyages" has had many different endings in different alternate universes, including ones where the casualties among the main cast ranged from zero percent to one hundred percent.
And so Trip survived to the end in many alternate universes.