Info.plist is always readonly
That file is explicitly protected by SIP.

ls -lO /System/Library/CoreServices/Finder.app/Contents/Info.plist


The "restricted" flag listed by the O (capital letter o) option to the ls command shows SIP status of the file.

To change that one file, you would need to disable System Integrity Protection. SIP is a security measure to prevent any User up to and including root (and thereby most malware) from modifying system files. 

To disable:


Boot into Recovery Mode (Hold Command+R while turning on Mac).
Open Terminal (Click Utilities > Terminal). 
Type: csrutil disable
Restart your computer, and SIP will be disabled.


You should now be able to modify Finder's info.plist file.