Owen,
You are correct in that RPKI leaves many problems unsolved.
One that it does solve is prefix splitting.
If I issue a ROA for prefix 10.1.2.0/23, any announcement of 10.1.2.0/24 (including mine) will be declared INVALID, because that announcement is covered by the ROA and the mask length is longer than maxlen.
Of course, as you rightly point out, if I do NOT announce that prefix myself, then anyone is free to announce it anywhere and have it declared VALID just by prepending my ASN.
Regards,
Jakob.