According to the Lifecycle of PowerShell Core, one has 6 months to update to the new version. However, during that period I still expect to be able to use 6.0 and 6.1 side by side, which is not the case for the Windows MSIs.
Installing 6.1.0 will remove 6.0.4 as part of it.
In the same way, 6.0.4 cannot be installed when 6.1.0 is already installed:

Again, I suspect this is due to the refactoring that happened for the 6.1 preview MSI builds to not have them side-by-side
cc @SteveL-MSFT @joeyaiello @TravisEz13