That's not entirely true - in the first place when he joined us, he replaced all of our players with players who were or become better than the previous ones. And sure, you can say that this was down to our "transfer committee" more so than Klopp, but the structure hasn't changed so drastically that all of a sudden all of our 'new' signings are Klopp's whilst the old, succesful ones (Alisson, Fabinho, Salah, Mane, ...) were not his.
Also it was basically impossible to replace Mane with a better player. I actually think Diaz is close as a replacement and one of the best ones we could've gotten.
A rebuild doesn't happen overnight or during one offseason. We went to the final in all our cup competitions last year and came within 30' of the PL title. At least you (although I can understand while Utd fans won't) should give him some time to properly rebuild. And that's a process of multiple years.
Arsenal stuck with Arteta during their losing runs, and you see what they're doing right now. Clubs are way too feeble these days. So what if Liverpool has two or three down years, we should trust Klopp that he can turn it around.