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?
At the moment, not. but It's just a consensus thing, and could "easily " be changed.
The divisibility of Bitcoins is how the lost coins problem is handled.
In short the money supply can be expanded by having the value of Bitcoin increase such that people will be urged to spend/sell to those the increasing demand.