The Zodiac Killer and D. B. Cooper are notable exceptions. And there are probably a bunch more.
It seems statistically unavoidable that there are always going to be some big cases that go unsolved. It'll probably become less and less common - especially since it seems inevitable that nearly all DNA will eventually be traceable via genetic genealogy databases - but some crimes just won't result in any DNA or other significant evidence being left.
If the case is high profile enough, they will _always_ find someone to find guilty.