The short answer is that Philip (or Phillipe*) wasn't hesitant about William's conquest of England, because there was nothing he could do about it. I imagine he wasn't very happy about it, but not because William would become independent: in 1066 Normandy was totally independent from the French crown's influence and its duke, a 38-year old veteran commander, was in no position to lose it, particularly to a 14-year old king who hadn't yet done anything at all of significance.

To understand why this is, let's take a bit of a closer look at French politics in the 11th century. While not as weak as some historians would make them out to be, the early Capetian kings ruled through alliances and influence, rather than by direct military might, as primus inter pares ('first among equals) amongst the great French princes (of which the Duke of Normandy was one). Up until 1051, the Normans were the French crown's most important and powerful ally: William's grandfather Duke Richard II was particularly important in Philip's grandfather King Robert II's Burgundian wars in the late 1010s. William's uncle Duke Richard III had been married to Philip's aunt, Adela, in the 1030s. When the illegitimate William became Duke at 9 years old in 1037, he was supported in his precarious position by King Henry I, Philip's father, who continued his support until after William had consolidated his rule in the duchy with his first great military success at Val-es-Dunes in 1047.

The relationship turned sour in the 1050s however, and William's increasingly expansionist policies, such as attacking Maine, prompted royal campaigns against William in 1051, 1054 and 1057, all of which the Normans successfully repulsed. After Henry's death in 1060, and the accession of the 8-year old Philip, William had pretty much free reign to do whatever he wanted, and, although Philip might have disliked it, there wasn't much he could do.

Relations between the Normans and the French kings remained cool right up until the French reconquest of Normandy in 1204. Philip took the first opportunity given to him to strike at William, but that didn't come until 1077, when William's eldest son Robert came to Paris seeking aid in rebelling against his father. William ended up buying Philip off by giving him part of the contested Vexin region. In fact, as far as we know the two men met in person only once, in 1079, at Gerberoy on the Norman-Picard border, but we know next to nothing about the context. But, as was the case with the conquest of 1066, there was probably unease and suspicion on both sides, but no open conflict.

*Conventions on names in medieval history are weird, but they're usually translated into the English equivalent. So we have Philip I, William I and Henry IV in the eleventh century (not Phillipe, Guillaume and Heinrich) but Louis-Phillipe I, George IV and Wilhelm II in the nineteenth (not Louis-Philip, Georg and William). I'm not sure there's any logic to it, but it's weird to think about.