Belgrano's guns mattered after her missiles were spent

Why was General Belgrano a serious Falklands threat when the Argentines also had Exocet missiles?

Missiles were scarce and quickly expended, while Belgrano's guns could keep firing and inflict sustained damage.

Endurance of fire is the key distinction. Exocet was dangerous but finite, whereas General Belgrano could continue using her guns against ships or shore positions so long as ammunition and access permitted. An older gun cruiser could therefore remain operationally consequential in a missile-age campaign.