Different for different people. My sister trades vehicles every 2 years regardless of miles, usually under 20k. My vehicles I'll run over 250k. I have never had to replace transmission. I have replaced a motor in a Ford Diesel 6.0 at 263k. Just last week I had the head gaskets replaced on a gas 5.4 at a cost of $3200. Previous owner had put in a rebuilt motor but the head gasket leaked on the drivers side.
I have a truck fund that is invested in the market. I make enough each year to pay for any repairs and my fuel for the year without touching the principal.
Another factor is the cost of personal property tax, sales tax and insurance on a new vechicle compared to a older vechicle.
I joked with my mechanic when he retires I will have to start purchasing new vehicles again.