As mentioned already, I wouldn't call an HDEO interchangeable with a break in lube, a pure racing oil, or VR1. It is, however, another very good option for various circumstances. I wouldn't recommend that someone building a performance engine go and grab it and use it for break in. On the other hand, I'd have absolutely no hesitation about using it in my F-150. Additionally, 15w-40 got a lot of use in the old Audi, as it was the widest recommended viscosity. People who do have flat tappet high performance engines should be cautious, particularly at break in. But, the guy driving an old slant six or a Ford 300 doesn't need to go and buy a racing oil or VR1 20w-50.
In older engines, too, there are other things we have to consider. An older engine can benefit from detergency, either from an HDEO or a modern GF-5 oil. In carbed applications, fuel dilution is certainly a possibility. In my F-150, it was terrible before the rebuild and carb replacement. To maintain operational viscosity, I had to use 15w-40 (for the summer) or a non-ILSAC PCMO (MaxLife 5w-30, non-ILSAC at the time). Otherwise, the oil light would come on.
While not all carbed engines were as terrible, we have to be cautious when comparing a carbed example versus a modern fuel injected system. The introduction of fuel injection probably did more to extend OCIs than most other advancements. We shouldn't underestimate the issue of fuel dilution; look at some of the DI examples we see today.
When it comes to detergency competing with anti-wear, we see oil companies trying to strike a balance. This is nothing new. After all, break in lubes and racing oils have little to no detergency for a reason.
Note the GF-5 oils aren't exactly light on the detergency. I would say that few of us here are equipped to determine whether a GF-5 or an HDEO or a racing oil has the best anti-wear properties for a given situation, unless that situation is really obvious (i.e. GF-5 in a new flat tappet race engine is silly, and race oil in a taxi is pretty foolish). Generally speaking, though, to determine which (between a 5w-30 SN/GF-5 and a 10w-30 HDEO) would provide the best anti-wear in my F-150 or (between a 5w-30 SN/GF-5 and M1 0w-40 and Delvac 1 5w-40) my G37 would require a serious undertaking.
I'm aware of the fuel economy issues when comparing a GF-5 rated lube (such as a 5w-30) versus something like a 10w-30 HDEO, 5w-40 or 15w-40 HDEO in something like my G. And yes, it's false economy to choose a thicker, yet cheaper grade. But, that's not exactly what it's all about. Walmart Canada is nuts, and I mean certifiably insane, and sometimes it's necessary to vote with one's wallet. Imperial Oil seems to reward HDEO purchasers, and again, it's necessary to vote with one's wallet.
For idle and performance, I don't notice a difference between the HDEO and the PCMO. I've already gone through the fuel economy in another thread here, not to show that there's no difference, but to show that it's unnoticeable. Yes, it's there. Yes, I would agree that it costs me more gas. But, from an operational perspective, I wouldn't notice an oil switch unless it were 20w-50 in our -40 or something similarly odd.
With respect to ILSAC type lubes, I have nothing against them. I used about 500 bottles of QS conventional in a 1981 Impala over the years alone. That's one taxi out of many. My LTD went half a million kilometers and only saw QS and GTX. My Town Car was similarly treated. I simply had some Delvac 1 ESP in stock, and tried it. I may want to run some UOAs for the heck of it. I may want to stick with it, given the pricing. Of course, my mixing phobia is well known here. If I have Delvac 1 in there, I have to stick with it until I'm sick of it.
But, I hold no illusions. I would have an exceedingly difficult time proving that this choice is "better" than Formula Shell, PYB, QS, M1, Mobil Super, Valvoline, GTX, or whatever alternative we wish to pick.