Blizzard’s battle.net client (and many other competitors’ comparable launchers) are well programmed by now to download the new assets/exectuables and then cleaning from your harddisk (or SSD) the old unused assets/executables. There is no need of fearing “old bugs being kept” (as long as the bugs are fixed, they will be patched no matter you use a clean install as you used to do in the 20th century, or just via the new progressive patching system). So why wasting precious time & energy by deleting & downloading again the same audio/video assets?
We’re in 2023. Even classic installers will overwrite old files from previous installations.