Its one of reasons why physical ownership is great too. So all i had to do was find offline installers from Blizzard online, copy it to external harddrive and have my CD key be saved in textfile. But thankfully since i own original copy of Reign of Chaos and The Frozen Throne and in the manuals i have original CD keys. For example, there is the thing with Blizzard who made Warcraft 3 Reforged, and i didnt like how the menys looked.
I also love that Exe files or executable files is more independent from one software.