LOL, see you had posts hours later, guess you didn't make it?
Ok, I thought about this a bit more and actually I see it working out well. I'll explain with the half-orc wizard example. This half-orc doesn't spend time doing much of the things other half-orcs do, such as mostly physical activities. As such, he spent more time developing his intellect. It was harder for him because it isn't typical of his people. So, he traded in his STR +2 for an INT +1.
So, the idea is while (maybe) biologically disposed to being stronger, the PC decided to instead work harder to develop something else.
The halfling, instead of playing dexterous games like throwing rocks, etc. spent his time climbing and working harder at more physical activities. He traded in his DEX +2 for a STR +1.
Now, I am a purist as you said, but I don't mind this trade-off because I know it represents a sacrifice by the PC to break away from the norm.
Finally, I totally agree that the DM decides for their setting. If that breaks away from tradition by having super smart orc, high CON elves, etc. I am perfectly fine with that of course! As long as such things are covered in session 0, no worries. But, 5E D&D has a standard design, and unless the DM decides to deviate from that design, I think it should represent the norm.