I'm a believer as well.
Probably harks back to the 60's when engines just about made it to 60k before a rebuild would be the norm. Theory was the better you ran it in, the longer would last.
Over the last 30 years I have bought many brand new vans for business use, and in the early days I had an appointed driver who was "old school" like me and I used to give him any new vehicle to run it in. In the last 2 years I have bought 6 new vans and haven't driven any of them. As my business grew however, I was no longer buying vans one at a time but in bunches and had to resort to passing them direct to the appropriate drivers. I found in time made no difference, even after 150k the engines would run like a sewing machine.
When it comes to my own personal car however, I would treat it with kid gloves for at least 1500 miles even though I replace my car before it gets to 40k.
Another thing I grew up with is an oil change after 1,000 miles, that is a thing of the past as well. I feel however there is no harm in an early first lube replacement.
Alex