AUM, managing high wealth clients, running their own funds with expense ratios (also some of the lowest in the industry), uninvested cash, etc... Retail trading is commoditized now.
Anyone who really cares about spreads will be using limit orders. Otherwise you're talking about pennies on highly liquid shares.
The fact that we're even discussing the possible spread differences between market makers shows just how commoditized retail trading has become.
What you described is how you bet against retail traders. The bet is that they have no edge so it’s safe to run tight spreads and nice pure market making algos that assume random behavior at volume.
Nope, this is one of the counterintuitive things about people paying RH for order flow. Market makers can offer tighter spreads when they know it’s a pool of dumb money.
Low cost brokerages mostly earn money from the interest rate differential, ie what they pay from your un-invested balances vs what interest they pay you.
They also earn some money from 'payment for order flow'.