You're basing your approach upon human understanding that believers cannot be elect and lost. This is correct, however not in the manner in which you are showing. The pronouns shift from others to believers in verse 9.
Therefore, the only consistent interpretation for the delay of the Lord's return is that there are yet the unborn who will be saved, that when the last of the elect are saved, then the "day of judgment and destruction of ungodly men" will happen.
Again, I will post this copy from a near post.
The NASB gives the translation in this manner:
3Know this first of all, that in the last days
mockers will come with
their mocking, following after
their own lusts,
4and saying, “Where is the promise of His coming? For
ever since the fathers fell asleep, all continues just as it was from the beginning of creation.”
5For when
they maintain this, it escapes
their notice that by the word of God
the heavens existed long ago and
the earth was formed out of water and by water,
6through which the world at that time was destroyed, being flooded with water.
7But by His word the present heavens and earth are being reserved for fire, kept for the day of judgment and destruction of
ungodly men.
8But do not let this one
fact escape
your notice,
beloved, that with the Lord one day is like a thousand years, and a thousand years like one day.
9The Lord is not slow about His promise, as some count slowness, but is
patient toward you, not wishing for
any to perish but for all to come to repentance.