I maintain several Windows 10 systems and there seems to be no consistency when it comes to which ones have the old store and which ones have the new store. Right before posting this thread my NUC9i9QNX just received the new store update and this install is years old (and always on the latest Windows 10 build). Other systems that have far more recent installs seem to have gotten it much earlier. Just a curiosity.
i'm going for the latter (as in "random") - also depends upon the age of the computer/machine (not about when Win10 was installed)