@Count:
Usopp and Franky's internal conflicts would be different, I will concede that. Although this still retreads too much when it comes to the crew needing to build a new ship for my taste, especially when Oda will never top how emotional everything from the Luffy vs. Usopp fight to Merry's funeral was.
But how exactly will Oda make the plot structure for Franky's dream disillusionment work differently than what we already saw in the Water 7 saga? We even already knew that Franky gave up on his dreams for years until the Post-Enies Lobby arc. Why retread that again with another ship build requiring Adam Wood unless a cool new twist is added to the story dynamic?
There's only two ways I see Oda going about this type of story. Franky leaves the crew and maybe meets his pirate parents for a conflict formula we have already seen several times, or Franky gets depressed for most of an arc until finally cheering up somehow at the end of it. Both of those routes sound boring and redundant to me. That can already delve into most of the themes a destroyed Sunny would delve into, but have the dynamic of Franky's dreams hanging onto a thread than being completely shattered. We can already get a decent internal conflict for Franky out of not knowing how to repair the ship until maybe hearing that Adam Wood is in Wano Country or Elbaf. And it would have a more unique resolution than being a whole new ship AGAIN while still maintaining the quality in how he achieves his dream with a ship that has made it through thick and thin since his joining arc.
If the Thousand Sunny gets destroyed, Franky loses. He doesn't just stumble, he has to start back from square one and permanently live with the fact that his dream ship could not sail across half of the Grand Line, technically a whole ocean. Only a quarter at best. That is a permanent stain on his record he can never wash off, he has to settle for a consolation prize instead. That is not what One Piece is about.
The problem here is that the Thousand Sunny is the best way to make Franky's accomplishment look the most impressive. Building a new ship only dulls that because the Straw Hats are never going to turn around and sail through all of those islands from Water 7 to Totland again, barring the Final War maybe. Oda wants his main characters, no matter what their struggles and occasional defeats are, to look as impressive as possible. The Thousand Sunny being destroyed will inhibit that impression, not empower it.
Oda is telling the Straw Hats Pirates' tale, not just Luffy's. Luffy is the biggest focus, but that focus is still shared with nine-ten other characters. Evidenced by your very argument about Franky potentially having his own character development arc centered around the Thousand Sunny's destruction. The latest Shonen Jump magazine cover does not have only Luffy. It's Luffy, Nami, and Chopper. One Piece is about the crew, Luffy is only the biggest focal point.
And yes, the Straw Hat Pirates will look inferior. Nobody will call them inferior in-story, of course. But for us as fans, the Roger Pirates would still have the top ship for a Grand Line voyage. This is not just about the Straw Hat Pirates surpassing the Roger Pirates in one way or two, but matching or surpassing them in EVERYTHING they did. Oda clearly wants to do that because that makes for a more exciting story, and he loves to have the story talk about how impressive the Straw Hats are and form comparisons with the Roger Pirates.
Do you think it's sheer coincidence that the story keeps making comparisons between Luffy and Roger, and the Roger Pirates exploring certain places the Straw Hats also reached like Skypiea? Or Mihawk and Yasopp being close companions with Shanks, who is inevitably going to fight Blackbeard one day? Do you think Oda as an author wants any of his main characters' accomplishments to look lesser compared to characters who were only active One Piece's background lore never seen on-panel? You might not judge or feel insecure about the Straw Hats not measuring up to the previous Pirate King crew, but Oda likely does. Many others in the fanbase probably would. I know that I myself would find it pretty awkward. Because why focus on this journey if the last one was more impressive?
There are only so many ways you can fail before it becomes impossible to match your predecessor, who the story constantly keeps comparing you to. Or achieve your dream in the most captivating way possible.
Those are all good points regarding this topic, but it all depends on the quality and ingenuity of the execution if Oda goes through the route of destroying the Sunny. I'm not promising you that this would be a great turn of events, but that there's something potentially interesting. The only thing I just don't agree with you is that the only possible outcome of this scenario is making Franky look like a lesser Tom, because who knows what's in the future beyond this moment of loss. Once again, all lies in the execution.
My main argument here is that this route could bring new grounds for character development and themes in One Piece instead of being just a "déjà vu". I'd even argue that Franky deserves a great development for himself and this one could be an excellent starting point because of how intense and relevant it is. In a side note, this incident also raises the stakes in the series, since the loss of something valuable is so inherent to an epic story and makes you wonder if everything is safe.
I apologize for not giving you a bigger answer this time, but, in my opinion, too much of this discussion goes around speculation on the details of the execution. I'm just trying to be open to new developments that I think are possible. If you want my bet, right now my belief is that the Sunny will be alright even though highly damaged.
Nothing exists; even if something exists, nothing can be known about it. Even if something can be known about it,
