Radak: The Chesed that I did with him, to make him king, I will guard it forever.

Malbim: Also when the [good] deeds, for which I promised him kingship, will cease, I will strive that the Chesed and Emunah not depart. I.e. what Hashem does due to Emunah, i.e. the promise based on deeds, it ceases when those deeds cease. What Hashem does based on Chesed, even though it is not dependent on deeds, it will cease if someone more proper for the Chesed is found. However, what Hashem does due to Chesed and Emunah together [is never Batel, like I explained in Shmuel II, Perek 7]. E.g. Malchus Beis David was initially due to Emunah (David's good deeds). However, based on deeds, He was not obliged to build for him a Bayis Ne'eman, that kingship will be hereditary; this was via Chesed. I.e. He will prepare that his sons never falter from good deeds, so the Chesed will not depart from them. He will make them return to the good.

Radak: I will not falsify (annul) it.

Malbim: I will make the Emunah last; his descendants will always have good deeds (refer to 89:29:1:2).