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.
Indeed we can divide Bitcoin as to where we want.
However, I doubt that the coins lost due to the reasons mentioned above can affect the price of Bitcoin simply because there is no way the other Bitcoiners can know that another lost their wallet (hardware failure) or forgot their password. And even if they have heard about it, there is no way to prove that it is true.
So, unless people intentionally sent coins to 1DontSentHereCoz1tsGone4Ever they cannot prove that they are telling the truth or don't have any backups.