While "harder" drugs appear to give more pleasure while their effects are in place, their withdrawal symptoms are also that much more painful (e.g. compare withdrawal symptoms from cocaine with withdrawal symptoms from caffeine).

This doesn't hold in general, and in fact doesn't hold for your example. Cocaine has very rapid metabolism, and so withdrawal happens within a few hours of the last dose. From what I hear, typical symptoms include things like fatigue and anxiety, with anhedonia afterwards (which can last days to weeks). (Most of what is referred to as "cocaine withdrawal" is merely the craving for more cocaine.) By contrast, caffeine withdrawal often causes severe pain. Cocaine was initially believed to be quite safe, in part as a result of the absence of serious physical withdrawal symptoms.

Amphetamine and methamphetamine are probably the hardest drug in common use, so hard that Frank Zappa warned against them; withdrawal from them is similar to cocaine withdrawal, but takes longer, up to two weeks. Sometimes involves being depressed and sleeping a lot. As I understand it, it's actually common for even hard-core speed freaks to stay off the drug for several days to a week at a time, because their body is too tired from a week-long run with no sleep. Often they stay asleep the whole time.

By contrast, in the US, alcohol is conventionally considered the second-"softest" of drugs after caffeine, and if we're judging by how widespread its use is, it might be even "softer" than caffeine. But withdrawal from alcohol is quite commonly fatal.

Many "hard" drugs — LSD, nitrous oxide, marijuana (arguably this should be considered "soft", but it's popularly considered "harder" than alcohol or nicotine) and Ecstasy — either never produce withdrawal symptoms, or don't produce them in the way that they are conventionally used. (For example, most Ecstasy users don't take the pills every day, but only on special occasions.)

I agree that alcohol is old. However:

1) I can't tell if it's much older than others. The estimates I can gather (Wikipedia, mostly) for their length of time mostly points to “at least Neolithic”, so it's not clear if any is much older than the others. In particular, the “since Neolithic” interval is quite short in relation to human evolution. (Though I don't deny some evolution happened since then (we know some evolution happen even in centuries), it's short enough to make it unsurprising that not all its influences had time to propagate to the species.)

2) On a stronger point, alcohol was only available after the humanity evolved. Thus, as something that an addiction-protection–trait should evolve for, it hasn't had a lot of time compared to traits that protect us from addiction to everything else we consume.

3) That said, I consciously ignored alcohol in my original post because it seems to me it's not very addictive. (At the least, it's freely available, at much lower cost than even ten kiloyears ago, lots of people drink it and most of those aren't obviously addicted to it.) I also partly ignored cannabis because as far as I can tell it's addictive propensity is close to alcohol's. I also ignored tobacco because, although it's very addictive, it's negative effects appear after quite a long time, which in most of humanity's evolution was longer than the life expectancy; it was mostly hidden from causing selective pressure until the last century.

What sort of selection?