Playing Devil's Advocate here.
Never once do we get the impression this time with the Oberhausers was anything more but two weeks of holidays. And forgettable rainy holidays at that. Bond shows more emotion to the DB5 than he does to the Oberhauser story.
Bond loves his car. He doesn't share that sentiment when it comes to Franz. Bond tells Q "He's not someone I'll ever forget." Franz certainly made an impression on Bond, very likely a negative impression, but Bond never realized just how much of an impact he had made on Franz.
This is not convincing on any level. Not just is Bond immediately willing to accept a man is alive he would have good reason to assume dead for over 30 years - he also swallows the idea hook, line and sinker that this man is the head of a mafia-like crime organization. It's as if Bond has been looking for a Moriarty all his life. And thankfully finds one.
But Bond directly sees all of this with his own eyes. There isn't an idea to swallow. It's all right there. He sees that Franz is alive. He sees that he is the head of a mafia-like crime organization.
Finally, the motivation on Blofeld's part is just as ludicrous and it would have helped the film a great deal if Bond had at least addressed this. Franz kills his father out of jealousy? Because of a few weeks some boy, hardly more than a stranger, has spent with them? Are you serious???
It only takes a one night stand to ruin a marriage, destroy a family, and get someone killed. A young boy growing jealous over his father spending time with another boy over a period of months seems very plausible to me. Especially if that boy already had problems, something Bond alludes to, if not directly spells out, in his conversation with Q.
That should have been Bond's reaction, even more so after the claim Blofeld was behind the past events. Any sane person would ask Blofeld where he lost his marbles. Bond instead goes along with the freak show. Not because he's just as meshugge as Blofeld - or so we hope - but because the script demands it.
Bond already knew that Franz was messed up. He's not surprised he finally went off the deep end. Sane people usually do not run Illuminati like organizations.
As for Blofeld claiming he was behind EVERYTHING. As the head of Spectre and it's offshoots such as Quantum and Silva he is indeed pulling the strings but it wasn't always directed at Bond. In Casino Royale Spectre was simply cleaning up the mess that Le Chiffre had made and Bond exacerbated. Blofeld may have even been amused that Bond was involved but he remained in the shadows and didn't pursue a personal vendetta. However once Bond brought down Quantum Blofeld realized it was time to strike back at James and those he worked for. By unleashing Silva on MI6 Blofeld was able to embarrass MI6 and get M killed. It played directly into Blofeld's 9 Eyes scheme and Bond still would be none the wise, if not dead. Unfortunately Bond survived and Blofeld knew it would only be a matter of time before they would confront each other directly.
It's similar to what happened in the Connery films. Bond starts off as a "stupid policeman" who continually interferes with Spectre and Blofeld's plans until they finally meet each other face to face. I will grant you that EON could have accomplished the same goals WITHOUT the backstory of Bond and Blofeld having known each other as children. I chalk it up to EON not only wanting to do something different with the Bond/Blofeld relationship but also finding ways to explore Bond's past without creating flashbacks. Tying Bond's backstory with the villain's accomplished that. It's also very reminiscent of Tim Burton's Batman. "I made you, you made me first." Having Batman and The Joker responsible for creating each other was controversial at the time but has become less so as each new interpretations of Batman hit the big screen. I suspect that same with Spectre the further away we get.
That really gets to the heart of the matter you either buy it, or you don't. It never bothered me because I thought there was enough material in the script to support it but as you put it there's enough material or lack there of to also not buy it. So to each his own.
Anyway good discussion.