They have already taken millions from FEMA to fund ICE. In a world where the weather is becoming more and more extreme and many lacking their basic needs, it would be irresponsible for emergency funds to be used this way.
If he does all of this he opens a big can of worms. Others after Trump can use this "national emergency" power to do almost anything they might want also, including reversing Trump's insane decisions. Let's assume right now that Trump is given his wall money. It would be doubtful if a wall like Trump wants would be built be 2024 due to legal and land arguments. The Trump Wall is a very complicated matter.