Is there a reason to think that some who take the mark might not survive into the millenneum?
Don't think they will get in. I believe there is a sheep-goat judgement at the end of the Trib. (At the time of His Second Coming.) I don't think the cursed and damned to Hell will enter into His Kingdom. Verse 32 seems to be saying that they will be sifted out of "the nations". I would say that the nations would then only consist of the sheep. Under perfect conditions, only a few sheep could multiply pretty quickly.
Matthew 25
31When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory:
32And before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth his sheep from the goats:
33And he shall set the sheep on his right hand, but the goats on the left.
41Then shall he say also unto them on the left hand,
Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:
I believe that the following verse is also a reference to those goats (those who took the mark.)
Revelation 19:21
“And the remnant were slain with the sword of him that sat upon the horse, which sword proceeded out of his mouth: and all the fowls were filled with their flesh.”
Again, sounds like they wil be killed at His Second Coming and are destined for damnation in the Lake. Being killed at that point definitely precludes and excludes them from entering into the Millennial Kingdom.