Let me assist you with this...
Firstly, Willian and Lingard's respective technical abilities are on different levels to each other. Willian is a better dribbler, has a good touch and has good striking and dead ball ability. Lingard's touch is inconsistent, he can't beat his man and has a poor shot.
Even though Chelsea were poor last season, the overall team were still packed with quality aa they were defending champions. Willian was named player of the year for the defending champions last season. Lingard will never achieve that honour as he is too limited as a player.
Sure, willian is at his peak at 27, but people often forget that Lingard is no spring chicken himself. He is turning 24 in 5 months and has far too many deficiencies in his game at the moment to realistically develop into the player Willian is within 3 years.
I do agree that Lingard has good pace and workrate (as does Willian), but on its own, it's not enough for a top 6 side. Willian has other qualities that Lingard unfortunately doesn't, which is why he will end up with the better career.