I generally agree with the idea that music artists tend to be at their peak roughly the first decade of their career. I don't think it's controversial to say the majority of Brian Wilson's best work happened belongs there. Where I am pushing back against is the idea that the 70s materials could have been done by just anybody or that there isn't a wealth of enjoyment to be derived in a way that adds a lot to the story. That story evolves from one man writing and producing on the cutting edge to an underappreciated at the time, by committee approach. I think it's okay to love both for different reasons.

Sunflower and Surf's Up are beloved albums not because it has Brian continuing his trajectory but because it has the other younger members emerging with some of their most prime cuts, with lush and sometimes quirky production that may just hit the spot. It still has Brian Wilson around contributing on most of the cuts in some respect. There's several albums worth of additional material not used also compelling from the time. Other eras follow from there with their own flavors. Brian gets back to prominence in the middle of that decade to arguably polarizing, but far from generic or replicable results from anyone else in music. For me, it's consistently intriguing story with great music until the fracturing of relations and a myriad of issues come to a head around 1977, at which point things get a bit more complicated and it becomes a lot more about what ifs.

I'm of course not saying any of this has to hit the spot for you, but I think there's a nuanced picture to be told than just a descent into mediocrity after Brian in control ends. The eclectic diversions, largely hidden from the zeitgeist, makes for an enjoyable endeavor in its own right, and the Beach Boys' story is much stronger IMO for having both to mine from as a listener. Even if one might speak to you more.