This is actually a concern, in some respects.

To a certain extent the ETF is functioning like a second layer. However, in this case, unlike Lightning, no initial on chain transaction is required, to bootstrap oneself onto the layer.

The more liquidity held and transacted in this type of fee-less layer, the more miners miss out on transaction fees.

Yes, ETF product managers will have to make on chain transactions to balance their assets, but realisitically this could be done with a single on chain transaction per day/week/month.

I am not yet sure where the sweet spot is, in terms of alleviating fee pressure, and maintaining mining profitability.

Hopefully, the market will figure it out.