There are idiots who do not know the character Sanji, and do not know the true mindset of his personality and willpower.
The term ''Sanji has no willpower like Luffy and Zoro, so he won't be top tier'' is dumb and problematic on so many levels ...
I agree that, Sanji's willpower in getting stronger, obviously is not linked to his dream of finding all blue, he doesn't need to be strong to achieve his dream. Sanji's entire evolution of strength, development in willpower is driven to protect people and have the prominent role on the team, even being the wing that makes Luffy fly.
Sanji is like Whitebeard and Sabo. All their willpower to get stronger is to protect who they like.
This is very true, and goes to the essence of Sanji. And proof of this was on display on Thriller Bark(among many other instances) when Sanji told Zoro he had his dream to worry about while Sanji prepared to sacrifice himself and his own dream to Kuma. It showed two things proving your point: Firstly, it showcased that Sanji's desire to protect a crewmate is above the desire to find All Blue. Secondly, by doing that, he proves that what drives him to get stronger is in fact his protective nature, his kindness, rather than the dream of All Blue. Yet it's still a precious dream to him.
Every time Sanji made the choice to throw himself in the line of fire to shield another crewmate was a choice to fail in finding All Blue, but to succeed in saving a crewmate. How's that for strength?
Already in his introductory arc did Sanji put someone else above his dream of finding All Blue, and Oda focuses on this again in WCI arc, where he does the same. Sanji's introduction to the series was an exploration of his character more than his fighting prowess, we got to learn about his struggles, his principles, his promises,....
...his compassion:
His compassion broke through Gin's barrier and reached his core, as it did with Viola, Pudding, the Vinsmokes. These characters lived their whole lives without experiencing something similar, they thus underwent a change and Sanji turned them from enemies to allies. By showing some broken souls true kindness. Again, how's that for strength?
Sanji's strength/willpower is derived from his care for others.