I do admit one of Snake's defining traits as a hero is his vulnerability. We can see the age lines and sorrow on his face and there's something genuinely human about that. If he keeps showing up it's just going to make him seem "immortal" and to me that kills the best things about his characterization.
Yes, Kojima can justify his existence with another clone or even an android plot twist but when death no longer matters why should we care? Where are the stakes? Where's the true heroism of sacrifice?