Well, they lived very long lives, and there may have been a very low rate of disease, and therefore that would give rise to greater development. If someone lived for nine centuries, we surely do not think that at 100 years of age, they were deteriorating in the same way as we are at 100 years of age, and that they spent the next 8 centuries pottering about as though they were people near the end of life.
This union of the line of Seth with the line of Cain angered the Lord greatly. The world is borne with patience by the Lord for the sake of those who are vessels of mercy (Romans 9:22-23). But if those who profess faith become no different to those who are vessels of wrath, then there is no reason for patience to hold back God’s wrath any longer. The godly must keep themselves separate from the ungodly, and even in Old Testament times marriage was not to be an unequal yoke. Whenever this rule is not kept the outcome is predictable: the godly will become like the ungodly, not the other way round.