The 100 years can be changed to 200, 500 years, the time does not really matter.
I was thinking for the coins to be "free" to mine as a simple and elegant method of putting the bitcoins back into circulation; but if that's unfair to the bitcoin holders we can find another redistribution method.

True, lost coins are like a wallet or jewelry lost in the ocean. But, fiat can always print more and gold can be mined or found in set ocean. Bitcoins on the other hand, can be thoroughly lost forever.
Again, this is an approximation, but 4 million lost coins out of 21 million. That's like 20% of the total gold/fiat supply just vanishing forever.
But yes, this is not really the biggest problem for Bitcoin right now and a consensus change against Satoshi's view will never happen anyway.