COTS still needs to be installed and configured.
And to do that install manually could take a long time, and be error prone.
Take something like Documentum which can take around 1 week to install by following a massive Word document which tells you all the step-by-step instructions. Very easy to get wrong and very difficult to automate. You would be much better off choosing a package which can be automatically installed and configured with Chef, Puppet or Ansible. This is still DevOps.
You can forget about Continuous Integration for the COTS aspect, but if you are then further customizing you'll likely need that too.
As for Agile, I think you are conflating two different things here.
In the above scenario it would still make more sense to develop your Config Management scripts using scrum and in sprints, delivering a working solution at the end of each sprint.
Now, when it comes to SaaS your argument probably does make more sense. You aren't going to "do the DevOps" if you are using standard Salesforce.