I think Delta has had a tough time positioning Premium Select. The "shelf" for an intercontinental flight is getting crowded with Basic Economy, Main Cabin, Comfort+, Premium Select, and Delta One. I'd bet that if you asked a non-enfranchised customer to describe the products, they'd nail Delta One ("the best" / "First Class" (ugh)) but would really struggle to differentiate between some of the options in the middle. Delta brought this on themselves a bit by marketing Comfort+ as though it were a separate cabin (though it is not).
There are two key markets for PS: business travelers whose travel policies allow it (price-insensitive, will choose based on amenities) and premium leisure travelers willing to pay for a marginally better experience (price-sensitive, will choose based on price). The pendulum has swung significantly towards leisure since COVID, so airlines compete on price and don't bring back enhanced amenities to woo business travelers.
If premium economy was closer to domestic business class (food on china, more than one pass with the drink cart / a flight attendant to fill orders from the galley) I might be more interested in buying it. Right now, PS is just economy with a nicer seat.
premium leisure travelers are not going for PE. They are going for business or first these days.
They may be buying discounted business class tickets, but they are still buying business class tickets. I usually only buy J when traveling on leisure. The ultra wealthy leisure travelers are buying international F
Now the business travelers who are forced to fly PE (which is becoming very common as most businesses do not pay for J anymore), may not be the happiest about economy service