@FearUS Yeah that's basically my one issue with how this went down. Even If Ymir herself got moved by Armin and Zeke talk-no-jutsuing each other and the rest of the past titans, It shouldn't really matter because we got told the only reason royal blood mattered for the founder was because of Ymir's past as a slave.
Hell, look at Historia and how Eren only got the future memories from touching her only After she claimed she was royalty (implying Ymir has to recognize the person as such for that to work?? Because literally is arbitrary and doesn't actually matter). Some people pointed out how Historia randomly got memories from Ymir's letter in the anime (because supposedly Yams claims the anime is the final version of the whole thing, or something), which also was related to her knowing she was of royal blood and by extension Ymir knowing.
Anyways, It kinda doesn't make sense. I see it as the only way for the other characters to stop Eren from destroying everything, but is also a little jarring because is bending itself over backwards for it to happen.
EDIT: UNLESS when they say they need royal blood, is not refering to the shitty King Fritz, but to Ymir, who was the one who had those powers. Then It would make sense for the need of royal blood, because you actually need the blood of a Ymir descendant. But then again all Eldians descend from Ymir, so what gives.