Let's assume that Bitcoin doesn't die in a few hundred years from now on, and our descendants (or even us ourselves if we manage to live that long) could reclaim these coins (coins that didn't move all that time) as public property for the common good
If you're looking that far into the future presumably brute force hacking could have improved so much that the missing bitcoins could be recovered.
In fact, we don't need that
We don't need to wait till advanced brute forcing becomes available (through quantum computing or otherwise). Actually, we don't need to wait any time at all. All we need is consensus what to do with lost coins and consensus in respect which coins to consider as lost. If we have both we can easily retrieve the coins that we think as lost. We would only need to make a few small changes to protocol (i.e. the way Bitcoin transactions are processed). For example, if it becomes known for certain that Satoshi is dead (provided he was alive in the first place at all, of course), and no one is going to claim his coins (his heirs or beneficiaries), we could retrieve and share them among all active Bitcoin holders (or spend them on something useful)
Why would we need to vote something for what's already deemed as irretrievable? Voting to recover coins that are already lost is stupid and just makes available coins be less of what it's worth today. The key idea of a deflationary coin is to make available resources scarce over time to increase its value. Also, we aren't so sure if these coins really are lost or kept very well in cold storages
Well, there are other options
In fact, we don't actually need to get back the coins which are considered as irretrievable and lost for good. We could just raise the cap if we ever need that. If you are going to ask whether we will ever need that, I should just point out an important thing which almost all deflationary coin supporters invariably miss. The point is that scarcity by itself is not enough to make something valuable. That something should be useful at that, or rather, in the first place. So it is actually an inverted U-curve on a scarcity-value chart. If something is as abundant as air, it won't cost anything even if it is as vital as air. On the other hand, or rather end, when something is infinitely scarce it will lose value because it will be quickly made irrelevant. Say, there is no more air, therefore everyone quickly dies, and air loses all value soon thereafter. Whoever (or whatever) manages to survive without it will no more need it altogether, i.e. it will be of no value to them despite its infinite scarcity
As long as there are people who agrees that something has value, no matter how abundant it is, there would always be a price tag for it. I don't know if we're on the same page, but apparently manipulating the 21 million cap would defeat its purpose of limited availability, and I don't think that the hard-coded limit would ever be changed even after our lifetimes.
Also, people tend to find value/price on nearly everything that they see. I wouldn't be surprised if someday, water will be monopolized just like some basic necessities that we have today.
