Given the economics of SaaS where frequently revenues are dominated by the very big accounts, combined with large companies getting even larger, I can definitely see a world where a company with 10,000 sales people on $2k/yr Salesforce licences might roll out an in-house solution. (though even then, I wonder if $20M/yr is a big enough cost driver for such companies to take the leap)

At some point it makes economic sense to roll out your own data center. At some point it makes economic sense to design and make your own server and networking equipment. The question would be where that breaking point is for SaaS.