In the end, it seems that the main solution would be making bitcoin more divisible, in the event that one satoshi starts to become too expensive and too impractical as the smallest unit, and so if a decent amount of private keys are lost and continue to get lost (which seems almost inevitable that keys are going to continue to get lost), then whichever coins are left might need to be subdivided further in the event that there are not enough coins to go around or their are not able to be used in the transaction of goods, services and/or other reasons for transacting value.
This is what I would also say about a solution. We know that bitcoin's smallest unit is Satoshi but if there's a smaller unit than satoshi then it will surely be a good solution if the smallest unit is too expensive. I think it's possible to do that since lightning network have smaller units than satoshi which is millisatoshi which is much smaller than Satoshi as you already know that. I think it will happen on bitcoin main network if satoshi become too expensive and won't even be called as the smallest unit in bitcoin cryptocurrency.