I don't understand.
Do you literally mean just because that slow rate takes longer than, say 3C?
The slow rate is much better for longevity, especially in cool conditions.
Heat is hard on cells. If the cell isn't getting hot at 0.5C, it's fine to charge it at that current. Sure, you can charge it less, as long as your charger can properly -dV/dt terminate at a slower rate. But there's no need to.
I sometimes charge Eneloops at 1C. The do get quite warm at that, so I only do it if I'm in a hurry. So, 0.5C is a better rate. Lower doesn't harm them, unless your charger misses the termination.
> You can do it, but a healthy cell won't get very warm at 0.5C.
Of course, but that sounds like you would want them to get warm? Not me. . .
I'm not sure why you think I said that. I meant that if a cell isn't getting warm, then it's fine.
> Once a cell is old, worn-out, and develops high internal resistance
I should be replaced, right?
My EoL cutoff is 70-75% SoH. . .
If you intend to use it in high-drain applications, like flashlights, then yes replace it. Otherwise, old cells do very well in low-drain devices, such as remote controls (assuming they are low-self-discharge like Eneloops). High internal resistance doesn't matter if they're only being drained at a few milliamps.
I have old, worn-out Eneloops that have very high internal resistance. I can't use smart chargers to charge them, because they won't recognize them as NiMH. But I can use slow dumb chargers, and they charge up fine. They hold their charge very well, too, almost as well as when they were new. They just suck at anything more than about 100mA.