JKR has said that part of the reason she saved Arthur was that she needed Ron to continue to cheer Harry up. This is true for the Weasleys as a whole, I think. Ron grieving his father would have been interesting but would have distracted from Harry’s grief. Harry needed someone to pull him out of his own grief and Ron couldn’t do that if he was mired in his own. Not to mention the additional angst Harry would feel due to the death.
Though Remus was a great character, I personally never really thought that he lived up to his role in Harry’s life. He cared for Harry, but he generally kept his distance and did not reach out to Harry as often as he perhaps should have done (not contacting Harry when he started school, not continuing their relationship post-PA, not reaching out after Sirius’s death, etc.).
Given Remus’s history, I think that if Tonks had still died, Remus would sort of fall into old grief. He would be dealing with fresh grief and raising an infant on his own, and I don’t think he would have the energy to serve as a mentor for adult Harry. Even if Tonks didn’t die, and Remus was significantly happier than he was in canon, he has still sort of outgrown his usefulness. Within a few years, Harry will be forging a new path as a husband and later as a father. Remus is just a few years ahead of Harry and has a rocky road up to that point, so his ability to help adult Harry is more limited. And of course, James died at age 21, so stories about James are still important but no longer strictly relevant or helpful to actual problems in Harry's life.
On the other hand, Arthur is an older figure in Harry’s life that has been a quiet constant since Harry was twelve. He has seen Harry grown and develop into the man he is today. He’s also the father of Harry’s wife, and in a position to serve as a surrogate father to his new son-in-law. If Harry encounters a bump in the road in his relationship with Ginny or his relationship with his kids, Arthur is well-placed and experienced enough to provide advice and support.
So if we’re killing off father figures to Harry, I would say that Harry has outgrown Remus in a way that he has not outgrown Arthur.