In this case it's actually the other way around. There are good theoretical arguments that without junk DNA at least human reproduction would be quite difficult

In this case it's actually the other way around. There are good theoretical arguments that without junk DNA at least human reproduction would be quite difficult. The average number of mutations per human generation is known and most mutations are deleterious. They destroy something. The mutation rate is high enough that the genome would quickly deteriorate over multiple generation. But with most of the genome being junk most mutations don't matter, so this problem isn't that serious, and humanity survives.