The ratio of premium economy seats is not that much different for UA compared to their competitors:
UA fleet - 7 to 13% of seats are PE (lowest 787-10 21 out of 318 seats; highest 763 high-J 22 out of 167)
AA fleet - 7 to 12% of seats are PE (lowest 787-9 21 out 285 seats; highest 787-8 28 out of 234)
DL fleet - 7 to 16% of seats are PE (lowest A333 21 out of 282 seats; highest A359 48 out of 306)

Some airlines have even lower ratios, AC hovers around 6-8% of total seats in PE. Other more premium airlines such as BA has 18% of seats in PE on their 787-9. (And SQ A359ULR has 58% of seats in PE but that's a unique market).

Also UA, on average, has more J seats than AA/DL, so not sure if they were initially hesitant that PE might cannibalize their J fares.