How would they even do that? Bitcoin handles ~350k transactions per day. VTI (Vanguard's Total Stock Index), a large size fund is on average traded 3,670k per day. That's one fund. There is no reasonable way for small customers to pull out of index funds to bitcoin unless there is a fundamental change to how bitcoin operates.
Small customers get BTC from an exchange, which keeps their accounting internally. No transaction volume limits.
Later the small customer may move that BTC to self-custody, which could be an on-chain transaction or LN or other side chain with more transaction capacity.
