Why would Beowulf's status as Hrothgar's "adopted" son have caused concern amongst the Danes?

Although it is impossible to know exactly what Hrothgar means when he says that he will love Beowulf "as mine own son," as many scholars have pointed out, the statement may have caused some concern among the Danes, especially Hrothgar's queen, Wealtheow, because Hrothgar already has sons, and the statement could be construed to mean that Hrothgar will treat Beowulf as his son, perhaps his eldest son, who might inherit Hrothgar's throne. It is most likely, however, that Hrothgar means exactly what he says—that Beowulf will be loved as a son, not take the place of Hrothgar's natural sons. We can assume, though, that when Hrothgar uttered these words, every Dane at the celebration paused for a second to look up at Hrothgar. Wealtheow makes it clear a bit later that she will entrust the care of her sons to Beowulf as their protector and essentially warns Hrothgar not to do anything that might put Beowulf in Hrothgar's line of succession.
https://www.owleyes.org/text/beowulf/read/xiv