Can Hagar bring forth? Of course she can; And did.
But, what God says, He means. Abram could've bed-down all the Hagars he found among Sarai's handmaidens, but God did not say Hagar, He said Sarai. Therefore, Hagar brought forth Ishmael. And indeed Hagar could've produced a whole quiver of Ishmaels, but only Sarai was anointed to bring forth Isaac, the promise (the reward) upon whom God's covenant would rest.
Sometimes God has to rename or even reconstruct a thing in order to preserve the integrity of His name, at times even commanding unto the remnant, a new covenant. Whichever the case may be, God is not mocked, nor will He be made a liar, for any reason; certainly not because of man's rebellion or ill-advised choices. Sarai and...