You can certainly make the upgradability argument ... custom builds are more friendly to upgrades.
You can also make the argument that with multiple brands competing for each part there is a higher market incentive for quality. The existence of sites like this one drive companies to establish reputations for quality -- all of the most successful brands are recommended by experts on this site. Buyers are actively discouraged from purchasing from vendors without quality reputations.
I think you are going to need to go with anecdotal evidence for your argument though - look for some experts in the field that you can interview. Maybe a community college tech professor or local business CIO?
It depends what parts you use in custom builds especially the power supply, you can put a cheap or underpowered power supply in a custom PC and it will only last a week.
You can put a cheap PSU and blow it up on first try.
I personally have been fairly lucky with my $15-20 PSUs. Aside from having had to change their output caps due to leaking which caused the PCs they were powering to start boot-looping, they have been performing flawlessly ever since. One of them has been powering my 1GHz P3 24/7 for most of the past 10 years and still counting.