These buses will still be around for a number of years even though the BMTA is attempting to replace much of the fleet. The BMTA has asked the govt to provide funds to update the fleet for around 12 years now Including a request for 5000 new aircon and non aircon buses after the junta came to power. At any given time around 800 buses are out of action due to age & maintenence issues) All that has been funded was the 490 NGV Bestlin buses a few years ago, but there is obviously a push to electrify the fleet.
As fares are charged according to the quality of the bus, the non-aircon buses are the cheapest (8 baht) for the poor, min half the fare of a new NGV Bestlin bus (15-20-25 baht).
It is one of the crazy and confusing aspects of BKK buses for tourists. You can have 4 different types of buses with the same route number (BMTA non-Aircon, BMTA aircon, private operator non-aircon, private operator aircon) all with completely different fare structures.