Honestly ,since we are at the endgame and we saw last flashback of Rocks that old Legends like Roger ,WB and Rocks couldn‘t even scratch an Gorosei possessed by Imu .
There had to be someone like Loki showing up to even the odds .
You have to look at it like this as well.
Imu is so busted that mere HakiBlooms won‘t cut it anymore i guess
Let be honest, from what I saw of Roger and Xebec, nope it doesn´t even come close to that what Loki pose.
Strong Haki
Strong Physical power(surpassed his dad one).
Insane strong weapon(Ragnir)
Insane strong legend devilfruit power
Is massive big as a island
His Hybrid form destroyed HK Harald easily
Can most likely use Elemental skills(Ice/Lightning and more)
Most likely can use also soul manipulation
Special eyes (?)
Vs
(just example Xebec/Roger):
Haki.....
Oda can´t made me believe, pure haki fighter like Xebec or Roger, beating guys with so insane powers, just with haki.
And from what we saw with haki, is unbelivable to think it because haki doesn´t have spexial super hax skills like rejiatsu or nen or chakra...
Loki being stronger then guys like Roger/Xebec make sense.