> we inherited the practice from PayPal

Paypal got it from eBay, which in 2000's was rolling out 20M LOC worldwide every week or two on "release trains". There, a small team of kernel engineers rotated doing the merging -- two weeks of clearcase hell when it was your turn.

And, since eBay wrote their own developer tools, you'd have to deploy different tooling depending on the branch you were on. But because of their custom tooling, if there was a problem in the UI, in debug mode you could select an element in the browser UI and navigate to the java class in a particular component and branch that produced that element.