Wonderland opened in 1985 and LPS reopened in the aftermath of the Ghost Train fire in 1983. There was no void and both parks coexisted in the 80's until LPS shut down for renovations until reopening in 1995.
I get what you are saying though. I have no doubt that if the Big Dipper, the River Caves and Ghost Train had survived then they would now be heritage listed. However, the events of Luna Park Melbourne in the 80's are a cautionary tale and could have been replicated at LPS for the same reasons. In the aftermath of the Giggle Palace fire , the River Caves at LPM were deemed a fire hazard and were removed as a result. The Big Dipper was also pulled down in 1989 , even with a interim heritage order being placed on it. Luckily the 1935 Ghost Train/Pretzel ride survived and is now under heritage protection and one of the few remaining rides of that era left in the world. These events could easily have happened in the 80's to LPS , especially since amusement park history and preservation were almost unheard of, especially in this country.
The loss of the Big Dipper , River Caves and the Ghost Train is a huge loss to the fabric of the park. However, we now have a brand new, $30 million dollar investment to look forward to, that will propel LPS into the future for many decades to come. LPS will now be a fine mix of heritage , nods to the past and brand new attractions. The park that really should not have survived the many roadblocks of the past now has outlived and out shone its nearest competition to become a true Sydney icon.