I wouldn't say must. But I don't think Palu is so far ahead, Fotu should be disregarded as an option. Look at them equally, and pick who has performed the best come selection time.
Most people arguing for Palu are saying that Auelua should be on the bench to replace him in the last 20 minutes.
He was about our second best forward after Hooper on the EOYT. People that are arguing that he hasn't played good test rugby for years are delusional. It is fair to say that he's never dominated against the All Blacks, but who of our forwards have. We've played about 2 Bledisloe Cup tests in the last half a dozen years where our forward pack has been dominant.
Palu has clearly had injury problems over the years (and this season) but he was superb in his first game starting against the Chiefs. Obviously he has to stay healthy now leading up to the Lions series.
Meanwhile Auelua has probably turned in two of his weaker performances of the season in the last couple of weeks. He was particularly quiet against the Reds which was a huge game to stake his claim. He hasn't started a lot of games this year but it seems that when he does, his impact is less. Maybe he's a flat track bully when he gets to come on for the last half an hour when players are tiring. If that is the time when he is best, then perhaps he is best as a bench option against the Lions to really provide a point of difference (as Dingo would say) late in the game.