The C90 was probably quietly discontinued to boost sales on the 200…much like the Queen Air was discontinued to boost sales of the 90, and the Beech 18 was discontinued to boost sales on the Queen Air…
They successfully coexisted for 50 years. And the 200 is a notably larger aircraft. There was an obvious progression from the Twin Bonanza to the Queen Air to the King Air. The Twin Beech (18) wasn't discontinued until the Queen Air had been out for a decade and the King Air was out for 6 years, it just didn't really have a place in the product line any more.
What changed is not only the PC12 but all of the other single-engine turboprops that are not only cheaper to operate, but usually faster as well. The only thing the King Air 90 still had going for it besides the name was its reputation and the fact that it has two engines, which is becoming less and less necessary due to the PT-6A's increasingly stellar (and well deserved) reputation.
FWIW, Pilatus didn't join GAMA until 2001 so sales before that are somewhat of a mystery... But if you're correct, Textron's move was an utter failure:
In 2001, Pilatus sold 70 PC12s and Raytheon sold 41 C90Bs and 46 B200s (87). The two King Airs win by a bit.
In 2006, Pilatus sold 90 PC12s and Raytheon sold 52 C90Bs/C90GTs and 42 B200s (104). King Airs win by a bit again.
In 2011, Pilatus sold 63 PC12s and Hawker Beechcraft sold 29 C90GTs and 25 B200s/250s (54). Pilatus now selling more than the two King Airs.
In 2016, Pilatus sold 91 PC12s and Textron sold 11 C90GTxs and 32 250s (43). Pilatus now more than double the two King Airs, with the 90 series taking most of the hit.
In 2020, Pilatus sold 70 PC12s and Textron sold 8 C90GTxs and 20 250s (28). Last year of the 90s, which are being outsold by the PC12 by nearly an order of magnitude.
In 2021, Pilatus sold 88 PC12s and Textron sold 22 250s/260s. No real recovery in the 200-series King Airs.