This is exactly what I was thinking, though you said it better than I could. It's not impossible - just unprecedented.
From a 2014 signing: Question: Can a Parshendi bond a KR spren?
BRANDON SANDERSON: Historically, the Parshendi were not made Knights Radiant, or the parshmen weren't.
Question: Can they become squires maybe?
BRANDON SANDERSON: Historically they did not, but it's not impossible.
Also, the Listener Songs:
40th stanza of the Listener Song of Secrets - The betrayal of spren has brought us here.
They gave their Surges to human heirs,
But not to those who know them most dear, before us.
9th & 10th stanzas of the Listener Song of Spren - The spren betrayed us, it’s often felt.
Our minds are too close to their realm
That gives us our forms, but more is then
Demanded by the smartest spren, We can’t provide what the humans lend,
Though broth are we, their meat is men
But it is not impossible to blend
Their Surges to ours in the end.
It has been promised and it can come.
Or do we understand the sum? We question not if they can have us then,
But if we dare to have them again