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?
Lost coins reduce supply of bitcoins. Supply goes down, price goes up. You can divide Bitcoin however you want, so unless I'm missing something, it shouldn't be a problem.