There are a few things to keep in mind here:
- This will only be an issue for site/volume license holders of EndNote X7 who are a part of an institution that did not renew their license subscription. The renewal would update them to EndNote X8, which is Sierra compatible. Those who have a single-user license of EndNote X7 will not have this issue. Those who are at institution that does not renew their site/volume license subscription can also choose to provide the information so that end users can install using the single-user installation process to avoid this issue.
- There is absolutely no intent to “blame” Apple here. The changes to Gatekeeper protection were intended to improve security. For more information on the changes to Gatekeeper protection in macOS Sierra, here are a couple of links: https://9to5mac.com/2016/06/15/macos-sierra-gatekeeper-changes/and http://mjtsai.com/blog/2016/06/16/gatekeeper-path-randomization/. Due to these changes made by Apple, developers of applications for the macOS Sierra environment have had to update the way their software installs. As development for EndNote X8 was already in process, these changes were incorporated into that application.
- You can certainly run a command to bypass the normal macOS Gatekeeper security monitor for the DMG file. You can issue a command such as xattr -d com.apple.quarantine “EndNote X7 Site Installer.dmg” in the Terminal. However, there is no guarantee this command will function in the future should Apple choose to disallow it or modify the way the Gatekeeper functions.