I think Walz is a perfectly fine choice, though I'm not sure about what he delivers materially beyond even more enthusiasm from the base and probably very good Midwest/Rust Belt outreach (which is a material deliverable in and of itself, I guess). He was also allegedly Joe's first choice, as they hit it off recently at some bar or barbecue joint (oh, to be a fly on the wall).
Walz is a strong surrogate and very good in media. He is not a sonorous cadaver like Pence or a genial silhouette like Tim Kaine, he's got fire and delivers Beltway media's beloved have-a-beer-with vibes. My concern about losing Shapiro was he's younger, more photogenic and can likely lock PA. But we can get PA anyway IMO, and the issues with Shapiro may well have been seen as too much to overcome. This way, nothing (nothing known, anyway) stops the bullet train of hype we've been riding since July 21st.