When the fourth beast is removed, it says that the first three beasts are still there.
Dan 7-12. As for the rest of the beasts, their dominion was taken away, but their lives were prolonged for a season and a time.
A season and a time are 15 months.
This shows that these beasts exist at the same time. Not only at the same time but after the end of the fourth beast as well. Daniel 7 is not over a long period of time. What he writes happens in the age of Laodicea and the book of Revelations. It does raise some interesting points. How many people are left on earth after the Day of the Lord? For there to be enough people to be regarded as nations, and that would indicate quite a few. The only indication of numbers is that they would be as rare as gold. Gold is not that rare, every central bank has tonnes of the stuff in vaults. Some countries have thousands of tonnes of gold, and there is an endless amount of jewellery as well. Gold has value, like silver, not for rarity but because it does not corrode. A kilogramme of gold or silver is still going to be a kilogramme in a hundred years' time. A hundred million people left behind to start anew? That still means that 7.9 billion people are going to have their existence on earth ended. Although it could be that the saints who are caught up by Jesus just before the Day of the Lord go back to the nations they were taken from, Time will tell on that one.