The seat of Shichibukai isn't how one obtains One Piece, it can lead to it, but it's not. As soon as he got what he wanted, he left his position. He did tell his crew that One Piece isn't going anywhere, so it's quite clear he intends on getting One Piece and everything he's done up till now, is with that goal in mind.
And if someone is going for the same goal as Luffy, it means they'll have to fight eventually. Luffy even defeated Coby whom he considers a friend, and yet, is the man who promises to capture him; they're friends, but rivals at the same time. There's no reason why Law can't be such.
Law isn't one of the 3 main Supernova to simply become a fan of Luffy like Barto, Law is a man whose crew-mates expect him to reach great heights, not to be overshadowed or work with the premise of helping someone reach a goal which is what Luffy has his crew and fleet for; Law is not a part of this fleet, just an allied partner to take down a Yonko because that's what is required to find One Piece.
We can't forget this is an alliance, and alliances are made if the interest of both parties as met. As of now, everything is going well, they have increased their allies and it is ultimately leading to Kaido's defeat which was the goal from the onset, so the alliance is still on course as far as Law is concerned. He's still a Pirate nonetheless, and Law is a very smart one who plays a game of chess, and takes his time.
Even his meeting with Luffy was coincidence, nothing planned, and the fact that he was already working on his plan even before Luffy showed up, implies he was looking to form an alliance with any Supernova who wanted to take down a Yonko; Kid and co. only formed their alliance that very day, so whose to say he won't have joined them had fate not joined Law with Luffy?
Betrayal is still very much a possibility for me, and more so due to his position, it'd be awful to waste Law's potential on simply being another character rooting for Luffy, there's enough of that already. Law is also yet to live up to his epithet, he's supposed to be cruel, it's about time he does himself Justice.