What i am saying (or replying to and trying to suggest solutions for) is your assumption that they will be about equally common on paper, but since more shards are run than people farming for overworld legendaries, in effect there would be more on the market (therefore being of lower value than the old legendaries).
To solve this, they could be soulbound so that only those actually getting them would have them, and having more than 1 would be pointless other than perhaps a vendor sell price.
The other suggestion, in reply to still having the option to trade it without flooding the market with them, is to have them tradable with other legendaries.
In this way they will keep in line with either people getting the actual drops in either shards or overworld, rather since that is how they can be traded, rather than potentially “flooding” the market with them if the shard drop percentage is off compared to the overworld drop percentage versus the amount of people doing each type of grinding.
Lastly, yet another suggestion to solve the assumed future issue:
The shard legendary droprate could be adjusted in relation to the previous week’s total shards run. So after periods where MANY shards are run overall, the dropchance is lower than after periods where FEW shards are run to keep the overall rarity and value in check.