You understand that handedness doesn't dictate position right? All of Dermott, Lehtonen, and Sandin have played RD.
This is a really tricky point to get across for so many reasons but what you are saying generally is false.
1st, Handedness does not mean shoot, aka hold the stick so lets start there.
There are lots of people who are R handed but hold the stick left. It just feels more natural for them. So that is the first part that should be stated. The ability to switch and feel comfortable is a huge part of guys being able to play both sides. Not everyone can adjust to that.
Those who are playing their natural side having to switch over is extreamly difficult. Yes, they can play over there, but they have to change the way they play and the team plays to account for it. If you ever played you know it's really hard to backhand a pass with accuracy at a decent rate of speed on the ice. This means people playing their off side tend not to be able to perform point to point passes and that causes a huge problem for teams. See JJ on the RHD for the penguins 2 years ago.
What ends up happening for a lot of guys is they stop playing the "off side" and they play "center field" because it makes the passes shorter and easier to control.
In general guys who play their handed side naturally, do not switch well to their off side.
It's why guys tend to have an easier time going from LD to RD than from RD to LD. Because a fair amount of guys playing LD are actually Right handed. For them it's a matter of adjusting how they hold the stick, not playing the side.