There is a simple reason why i think Ryokugyu might succeed in capturing Luffy.
When previous Admirals were introduced, we knew the MCs had no chance so our expectations were that they are truly in trouble & we start to theorize on who's gonna save them? But this time it's the opposite, it's Ryokugyu who we don't expect to be able to handle the Alliance, so the plot twist wouldn't be who saves SHs this time, but instead Luffy does get captured indeed
It should also be noted that mostly when an Admiral is Introduced, even though SHs survive the encounter, something happens.
They survived Aokiji but his warning about Robin came to be true, they survived Kizaru but they ended up separated by Kuma, Luffy survived Akainu but he lost his Brother & they survived Fujitora but his warning against Shichibukai came to be true & now Hancock was most probably captured
Another thing to notice is that whenever an Admiral appears & start endangering MCs, another Major Villain appears shortly after.
Aokiji's Warning was basically an indirect warning about Lucci, Kizaru's appearance was shortly before Kuma arrived, Akainu's Rampage happened at same time with Blackbeard's arrival, Fujitora chasing SHs happened at same time Kaidou got introduced
So expect a Surprise Character to appear in Wano & make things even worse than Ryokugyu intended to
And very high chance that Luffy gets captured (Possibly others too)