I doubt Petro is intersted in signing in Toronto.
But regardless, if teams refuse to play their young D up the lineup then they should either send them down or trade them. Playing talented young D as 5s, 6s, or 7s generally just creates Ds who are 5s, 6s and 7s.
Boston has developed good D. In 2016 they started a 19 year old Carlo by playing him over 20 minutes a game on average. Later that same year they added McAvoy into the lineup - 19 years old - averaged more than 26 minutes a game in the playoffs.
St Louis has developed good D. When Petro broke in at 20 years of age he averaged 22 minutes a game. Parayko as a 22-year-old rookie was just under 20 minutes a game.
Anaheim has developed good D. In 2010 Fowler started at 18 years old playing more than 22 minutes a game. Lindholm a couple years later at 19 years old played just under 20 minutes a game. Manson was a late bloomer and didn't debut until 23 years old, but they ensured that he was playing over 18 minutes a game. Theodore couldn't break into the lineup becasue the Ducks D was so good, but when they gave him 19 games when he was 20, it was at almost 20 minutes a game. Montour didn't make the lineup full-time until he was 23, but when he did it was 20+ minutes a game.