The judgment passed upon Eli's family is not one of sudden, total destruction, but rather a decree of enduring tragedy and generational pain. Though spoken directly to Eli, the warning is actually directed at his future descendants [מצודת דוד, רד״ק].
God declares that He will not completely remove Eli's line from serving at His altar. While one might expect a severe sin to result in the total wiping out of the family, the primary approach among commentators is that keeping the descendants alive is actually a heavier punishment than death. The offspring of Eli will continue to serve in the sanctuary, but they will be reduced to a low and despised status. They will be forced to watch as the most important leadership roles are given to priests from a different family [מלבי״ם, רלב״ג, רד״ק, ביאור שטיינזלץ]. Living in the shadow of such humiliation is considered a harsher fate than extinction. This concept mirrors David's later plea regarding his enemies, asking God not to kill them immediately so that they might live to witness their own downfall [רד״ק, אברבנאל].
This ongoing existence will result in a hopeless, drawn-out longing for a greatness that will never return [מצודת ציון, ביאור שטיינזלץ]. The descendants will suffer the constant sight of others succeeding while they remain in poverty [מלבי״ם], bringing them deep sorrow and profound emotional grief [רש״י, מצודת ציון, רד״ק, ביאור שטיינזלץ].
Beyond the emotional pain, a tragic physical fate awaits the majority of the family, particularly those who are raised and groomed within the household [מצודת ציון, רד״ק, ביאור שטיינזלץ]. The commentators agree that these descendants are destined to die as young men, right at the peak of their strength [רש״י, רד״ק, ביאור שטיינזלץ]. The primary approach among commentators explains that the grief caused by the loss of a young adult is infinitely greater than the sorrow of losing a child; therefore, they are decreed to grow up only to be cut down [רש״י, מלבי״ם, מצודת דוד, אברבנאל]. Alternatively, others suggest that they will live just long enough to reach manhood so they can fully experience the bitterness and poverty of their lowly state in the priestly service before their lives end [רלב״ג].
This punishment operates strictly on the principle of measure for measure [אברבנאל]. Because Eli's sons enjoyed the honor of being the exclusive priests yet treated that status with contempt, their descendants are denied elders and leaders, forced instead to watch another priest rule over them. Because they sinned as young men who completely ignored their father's correction, their offspring are destined to die as young men. Finally, because they consumed the holy offerings prematurely and without patience, their own lives will be cut short prematurely.