Run this script on Cmd + W to prevent closing the last tab in Safari

keep_last_safari_tab_alive.applescript

tell application "Safari"
	set numWin to count of tabs of window 1
	if numWin > 1 then
		close current tab of front window
	end if
	if numWin = 1 then
		tell application "Safari" to activate
		tell application "System Events"
			keystroke "t" using command down
		end tell
		tell window 1 of application "Safari"
			close (tabs where index < (get index of current tab))
		end tell
	end if
end tell