what do you mean, "why?"
>stack sats onchain, several years later pay a $400 txn fee
or
>fees stay really low, several years later bitcoin has a security budget crisis
here's a fun experiment. imagine by chance the security budget crisis is averted because fees will be high enough to outstrip the block reward in a few halvings. I want you to guess the maximum size of an economically unspendable UTXO. good luck