Larry,
BY ALL MEANS you should use steel loads in your doubles. There will then be that many fewer good guns on the market. HA,HA!
You can believe what you want to Larry, but, none of my doubles needs to get steel shot run in it at this time. Key words being "at this time".

But, ask yourself if you know of a younger than 30 year old hunter ( I doubt it). And, then ask yourself what was the last gun that hunter bought.
I ran into "Kutter" (posts here) one time in the grouse woods up here, and I think he had a double-I had my Darne, and the retired super model along. But, I don't see people with doubles, as a rule of thumb.

Best,
Ted