A married couple under 30 just went on the Dave Ramsey show to celebrate paying off their $450K mortgage in Georgia. Ramsey turned it into a lecture about how 26-year-olds who say they can't buy homes should look at this couple for inspiration. The math on why this advice expires is worth running. They locked in a sub-3% rate around 2021. On a $450K mortgage at 2.75%, the monthly payment is roughly $1,837. On $145K household income, that's 15% of gross. Tight but manageable. Aggressively…