I don't understand why people think the Ryuma's grave plot is inevitable and MUST happen but they don't feel the same away about the "monster samurai" shit.
At this point, Orochi & Kaido are STILL right about the samurai.
Another thought on this:
Imagine if the quote was someone saying,
"We will never see another pirate as great as Roger!"
or
"We will never see another cook as great as X"
Everyone would
immediately recognize that the statement is specifically set up to be proven wrong by Luffy & Sanji respectively.
The "monster samurai" quote is clearly set up for Zoro to prove it wrong. Just saying.