The policies being cancelled are either cheap (or overpriced) junk policies with high deductibles and very poor coverage (no cancer, a couple $100/day hospitalization, or none, no diabetes, no CVD, no etc), giving the non-fine-print-reading policy buyers a spurious sense of security against medical catastrophe, and/or policies the insurance companies don't want to sell anyway.
About 20% of individual policies were cancelled every year BEFORE ACA (where's the Repug outrage?), so there's always been a lot of churn in that market.
Once these cancelled people obtain ACA-improved non-junk policies AND have a medical catastrophe, they'll be GIVING THANKS for ACA saving them from financial catastrophe, bankruptcy (no cc, no house or car financing for years).