Exactly, there's a pressure/flow/salinity tradeoff triangle. Higher salinity needs higher pressure and/or waste bypass. This was running around 100-150 psi, was

Exactly, there's a pressure/flow/salinity tradeoff triangle. Higher salinity needs higher pressure and/or waste bypass. This was running around 100-150 psi, was a few years back, don't quite remember. But a few hundred bucks will still get you from either "potable but not great" to fairly pure, or brackish to potable. If you have effectively infinite seawater and power, you can still pump at lower pressure and get pure permeate, it's just less efficient per unit pump and filter lifetime. If you had a super reliable pressure system (low friction ceramic pumps for example, not cheap but last forever) and cheap first-pass filters, you can run it for quite some time. But you're spot on in that you get to a point of engineering-give-a-mouse-a-cookie and it just makes sense to optimize the whole stack.