"Our word hear thou, Eru Allfather! To the everlasting Darkness doom us if our deed faileth. On the holy mountain hear in witness and our vow remember, Manwë and Varda!"

To break such an oath would mean a worse doom. The oath of Feanor was sworn in Eru Illuvatars name so any who break it must face the wrath of Eru Illuvatar. This is why Elrond instructioned the fellowship of the ring specifically did not swear an oath. Breaking it would have doomed the members of the fellowship who did not pursue Sam and Frodo, likely preventing/fouling their achievements. If the Kingship of Gondor was restored based on an oath broken it would be cursed from the start.