If this happens, maybe 401k providers will lose business. People often have reasonably large traditional 401ks, which they don't roll over to traditional IRAs,

If this happens, maybe 401k providers will lose business. People often have reasonably large traditional 401ks, which they don't roll over to traditional IRAs, because having traditional IRAs interferes with the backdoor Roth (and they don't want to do a Roth conversion, because that would be a taxable event). But if backdoor Roths went away, then there'd be no reason not to roll a traditional 401k into a traditional IRA. Brokerages like Fidelity and Schwab would win, as people brought their assets over, and 401k providers would lose, since now people would take their money out as soon as they switched jobs.