I don't think Arthur was completely sickly. It makes sense that he could have just been a teenage boy growing into his skin, but sickly or not, he certainly was not shaped like his buff brother. He seemed more like his thin, intelligent (though dark and suspicious) father. I think that Arthur would have been an OK king, not amazing or anything, but prepared nonetheless, because he got the education heirs' are supposed to get, unlike his brother Henry who got the education monks were to get. No one but God knew he was going to die . . . his brother was definitely not prepared to be King, because Henry was first politically schemed by Ferdinand and his wife Katherine when invading France and during Flodden (which proved fatal to their relationship) and then he took Thomas Wolsey as his right-hand-man while he enjoyed the company of Bessie Blount and Mary Boleyn in his bed.
