Cook linguine with tablespoon salt, adding peas in last 2 minutes. Reserve 1 cup water; drain when pasta just tender.
Meanwhile, whisk milk and cornstarch smooth in cold skillet. Add cream cheese, garlic powder, sauce salt, and pepper.
Warm over medium-low 4 to 5 minutes, whisking, until cheese melted and sauce gently simmering and lightly thickened. Lower heat if bubbling hard.
Remove from heat; gradually add Parmesan. Toss in pasta and peas, adding reserved water by tablespoon only as needed to loosen.
Serve promptly while light coating spreads easily along noodles.