Warband was also repetitive, frustrating, and extremely grindy
For starters, two wrongs don't make a right. Bannerlord is an unfun video game by the standards of games as a whole, regardless of what Warband is or isn't.
But that aside, yes: Warband's late-game was repetitive and grindy.
However, Warband was not frustrating and
extremely grindy and repetitive like Bannerlord is. For many reasons:
* The renown needed to be able to become a vassal was so much shorter that it didn't feel like a grind at all, and there was no arbitrary cap on the companions you could have, or being able to create a kingdom - RtR simply made it easier. Meanwhile, in Bannerlord, I can fight the entire population's worth of Calradia in doomstack battles and complete quests for everyone in the land, and still not be at Clan Tier 4 that allows me to create a kingdom.
* Towns and workshops provided consistent income that could support a war party and didn't feel like a complete waste of time and money and trap for new players.
* If you picked a friendly lord, raised your relations to 50+, recruited them and gave them a fief, you knew that they would stick around with you. But in Bannerlord you can pick a friendly lord, raise your relations to 100+, give them three fiefs and they will STILL abandon you. Now that's frustrating.
* In Warband you could kill a kingdom dead by keeping all their fiefs for enough time. It would cease to bother you ever again. In Bannerlord, you can take every fief from a kingdom and they will still magically generate the money to pay mercenary parties that attack your fiefs for eternity.
* War declarations made some kind of sense. You wouldn't declare war only to declare peace an hour later, just as you had traveled to a castle to siege it.
* You knew where people were heading towards. You didn't have to chase their last known location all over the map.
* Oh, and the map wasn't full of stupid bloody mountains. You could travel from point A to point B without navigating a maze!
* Warband had a bit more variation to the mid-late game gameplay loop than Bannerlord does. Both games have the loop of "fight battle, look at recruit menu, fight siege, look at buy menu". But Warband varied it up a bit more by also having you get surprise attacked entering towns at night, having to enter taverns where assassins or belligerent drunks could attack, hosting and attending feasts, getting called up by the marshal, the courtly love process, or fighting your way out after failing to sneak into a town. Companions would also talk to you about their backstory or petty feuds during travel, which is nice on the first playthrough and more immersive than an encyclopedia entry.
* Warband had bad balance, Bannerlord has shockingly broken balance where only 2 units are good choices, rendering the tactical gameplay absolutely null.
* The Warband map was smaller, so although the grind to conquer all castles was too long, it was not as long as Bannerlord.
* The player's combat mastery mattered more, due to more small-scale battles occurring and arrow damage not being pants on head stupid.
And much more.
Plus, the point of a sequel is to be an improvement. M&B 1 got a pass for its failings because it was the only game of its kind, created by a two-person studio that expanded to a ten-person studio. Bannerlord has had the benefit of 10 years of development, and hundreds of millions of dollars in funding, and is no longer the only game of its type. So it has no excuse to be an actual downgrade in many areas from Warband.