Ive seen plenty of stuff in 8 seasons to make me think the rules and boundaries only exist until they don't. They've shown enough to make me think a powerful entity could claim a vacant hell or heaven. With Kevin's translations being exactly that, slow going translations, and the tablets being books with inifinite knowledge and Naomi and Crowley both saying the tablets could destroy them I'm taking it at face value. The tablets do more than seal gates. Sealing the gates of heaven and locking the angels in isn't destruction. And if all Naomi meant was that angels on earth die when the gates seal then she could have just recalled the angels to heaven when Cas took the tablet and there would be no risk other than the gates sealing which again isn't destruction.
And you are also assuming we the viewers and the team hold all the answers about the endgame. In 8 season when have our heros ever seen all the cards before the finale? There is a twist coming or something much more substantial with these tablets. The gates sealing is just the tip. There will be a revelation that they got something wrong or a bigger plot is in play. At this point all we can do is speculate based on what has happened. That is what I am doing. I think the gates sealing is a secondary minor function to the main function of the tablets.
On another note, I wanna know what Naomi and Crowley got up to in Mesopotamia.