Someone like Phillips probably has to make the step up if he wants to keep his England place. There's so much midfield talent available for Southgate to choose from, and with more coming through all the time, at some point he's going to come under pressure to fit them in rather than persisting with a more negative approach.
Already there's Rice, Henderson, Bellingham, Ward-Prowse, Gallagher etc. to compete with for two positions. If Southgate decides to play with one holding player and two more advanced, Phillips is basically hoping to be picked as Rice's backup. Playing a decent amount of games under Pep and being part of an established winning team is more likely to make that happen than playing for Leeds week in, week out. Of course he could go somewhere like Arsenal or Spurs, probably play a lot more than he would here, but would that count for much?