I have several Shimano 600 bikes with six or seven speeds and downtube shifters. They work great just as they are. Reliable, quick shifting. If you ever need to replace the brake levers, you spend $30. If you ever need to replace the shifters, $20. Why spend hundreds converting to STI where a broken five cent spring in the brake levers may lead to a $300 tab for a pair of new levers?
Likewise, if you wear out a cog, your LBS can replace just that cog, rather than the whole cassette. A Shimano 600 system from the 1984 to 1989 era represents the all-time peak in the quality/cost/weight equation for a bike drivetrain.