You're conflating the romance of the outdoor rink, which I totally appreciate, and its value in producing skilled hockey players.
I doubt NHL players from the unrivaled hockey factory of the GTA acquired any meaningful portion of their hockey skills on outdoor rinks. I concede there are more outdoor rinks in the area than I expected.
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Toronto also doesn’t get the same type of cold weather that Western Canada and Finland do. It’s nowhere near as cold.
Outdoor rinks are a big part of the culture in Western Canada particularly on the prairie provinces (Alberta, Saskatchewan, and Manitoba) and parts of interior mainland British Columbia. It’s the same for Finland, the climate is conducive to that type of love of outdor hockey.
For Toronto, the GTA’s greatest advantage for kids is the quality and quantity of competition packed in a small geographic area. There are so many teams in the GTA that travel for competitive games is minimal. Whereas, if you’re from say Regina to play the best competition you’ll have to play teams from Saskatoon, Calgary, Edmonton, Kelowna, etc.