And I mean really, really lost.
A lot of people can't remember their passwords after 4 minutes since they set it up.
But apart this, there's several ways bitcoins can get lost, like in example if you have a wallet on a USB key and this breaks and you have no cold backup.
This will inevitably bring, in the future, to less than those 21 million cap.
And they WILL keep going down.
Is there a plan to reintegrate them?
There is no way to prove that a coin is lost, unless it was send to a burn address.. Most coins that are lost therefore, are not really lost, only forgotten. Reclaiming those would be like creating double coins.
It definitely would be and then the 21m cap would be broken. The cap needs to be one of the things that never changes with Bitcoin.