The circulation limit of the asset will not be just 21 million (value estimated by the system), it will be 21 million minus the bitcoins lost in wallets, which according to studies, is almost 4 million lost coins.
There are no "studies". There are less than 5 thousand bitcoin provably lost; the rest are just bareless speculation. 4 million is extraordinary, even with bareless speculation. Where did you find that number?
What could happen to these lost bitcoins? Will they continue to be lost in "limbo"
If you refer to non-provably lost (AKA, speculation), then either their owners will find access, or they will remain lost for a very long time. Those with public keys exposed can be brought into circulation when ECDLP can be solved within a reasonable time by quantum computers.