So my question is the following :
If I load some legacy addresses (paper wallet for example or vanity addresses ) with some low amount of crypto ( $50 to $100).
(They are going to be birthday presents for family members in 5 to 10 years).
When they will want to redeem the coins to a segwit or LN compatible wallet, what kind of fees are we expecting?
Can we expect tx fees to always stay in the $0.1 to $3 forever or are they going to follow BTC price and become (in USD) astronomically high?
Transaction fee is picked by you, not the other way around. A wallet might suggest a fee based on current network traffic, so that you won't have to wait too long, but that is
up to you.
You can ignore the wallet suggestion and force it use the lowest fee, currently 1sat/B. Such thing would do if you don't mind waiting hours or days depending, for the transaction to complete.
If you are going to make the paper wallets yourself, there is no reason not to make them segwit already. In fact you should make them native segwit.
Also do not try to see it in USD, because the price of bitcoin might be different in a few years. a few ¢ today maybe a couple of $ tomorrow.
LN I'm not sure what the situation would be...