Why Renown was too old to rebuild beside Vanguard

Could HMS Renown have been repaired and rebuilt to serve alongside Vanguard?

Not sensibly; her age and hard service meant another reconstruction would cost too much for the capability returned.

Renown was approaching thirty years old after service in two world wars and a demanding interwar career. Returning her to the line with Vanguard would require replacing or rebuilding much of an already exhausted ship. The costly reconstruction of Victorious provides the comparison: another rebuild was technically imaginable but a poor route to usable force. Preserving a famous hull would not buy reliable postwar capability.