they were received in 1982
several F-15C's have crashed , current F-15C's flight hours are huge and its probably gonna pass its lifetime (if it didn't already!) , very few F-15S's are in hangars , one F-15S crashed and one is damaged
these planes might need replacement for their position , and if the replacement is by the Typhoons then i find it better and excellent choice because it moves Saudi Air Force away from the US and the complications of its congress and israeli lobby to the opened European neutral market
the F-5's are now for sale at 1.5 million$/plane , some of them are sold to Brazil while there's been news about Kenya and Mexico to buy some. few F-5's are still at service in Taif as last i know about.
advanced MIG-29 , MIG-35 or SU-(of any new kind) option would be an excellent strategic choice (after what recently happened in the Congress by stalling the 20 Billion $ deal by israel lobby) and will secure the Kingdom against external pressure BUT technicaly its a terrible choice because it will damage the capability , experience and training of the RSAF and will cost much more money that damages the KSA's economy.