Cedar Fair did not kill Geauga Lake
That's a matter of opinion one way or the other. People will debate that for decades to come. You can definitely say that Cedar Fair had no intention of helping the park thrive. Their actions back up that statement.
In order for the park to thrive, they would of have to significantly reduce operating costs while still maintaining attendance. So essentially, remove rides but somehow still get people to attend.
The park was doomed as soon as SF added 4 coasters in 5 years with no plan of how to manage the park and for no reason other than to "compete" with CP. It was probably the dirtiest, worst managed, poorly up kept parks in the country. I believe the Geauga expansion by SF is probably one of the dumbest business decisions in the industry, possibly ever.
The additions combined with the poor operations drove away the local market for good and the additions were never strong enough to draw people away from CP. Remember, in the years SF owned Geauga, CP added Mantis, Millennium Force, WT, and Top Thrill. Absolutely no way a park could compete with that in a similar market.
If the park maintained its 80's and early 90's size and growth, it might still be around today. However, I believe the park eventually would have been marginalized by CP and Kennywood if it stayed small and not been operated with extreme care.