If pre-existing conditions were the primary cause for premium increases, the ACA's one-fell-swoop removal of them would've had far more significant impact on that chart.
Limits on pre-existing conditions mainly affect individual policy pricing, corporate health insurance are already pooled and because of that always had fewer pre-existing condition limits.
See Exhibit 1 at https://www.healthaffairs.org/doi/full/10.1377/hlthaff.2018..... The "sky high" jump you portray doesn't exist. It's been a quite steady annual rise.
If pre-existing conditions were the primary cause for premium increases, the ACA's one-fell-swoop removal of them would've had far more significant impact on that chart.