Egypt more than one occasion came close to doing something like that. Long story short. Who ever has better military capabilities will own the Nile.
Egypt can use Suez as another piece to get Europe to accept its bombing campaign.
Would they? GERD destruction = tsunami on the nile, possibly setting a chain reaction through downstream dams.
Sisi has now managed to beat even Murbarak on getting the lowest rating on record from any semblance of legit polls done nowadays.
Turkey is all over Ethiopia, and Sudan on top of that. And Sisi has a paranoid fear of Turkish involvement, either back home as a subversion, or a direct attack from Turkish African bases.
Turkey staring Sisi down from all sides is by far a much bigger concern to him than Ethiopians waving their MiG 21s at him.
If he has a gram of gray matter, he will not push Ethiopians further towards decision on Turkish military base.
Egypt can not use Suez, because the US, Europe and Israel will intervene for sure on that particular case against Egypt and Egypt will lose for sure.
If anything they will just dig a canal or create some route from Eilat port to Ashdod port and make some profit out of the whole situation.
IMHO, most likely result of an attack (for Egypt) is that Egypt loses a part of its airforce in return for temporarily harming Ethiopia's ability to generate electricity at the site. At which point Ethiopia fills the dam anyway and works on repairing the damage, while Egypt wouldn't be able to repeat this.
But yes, the planes would be expendable. That said, Egypt could very well breach the dam itself. It would needs some creative use of munitions but modern munitions are precise enough to allow for breaching of dams.