Side effects later on are unfortunate but irrelevant. As long as you make it to the age where you can reproduce, it's a win from an evolutionary point of view - conversely, dying before then is a major loss. I've no doubt that mortality rates creep up faster in carriers than in non-carriers later on in life, but the goal is to pass on your genes, which you can't do if you die at the age of 14 months. And once you've done that, your part is played. Evolution is about survival, not long and happy lives.
Yes, but that chart shows a big deficit for the HbSS. It does appear that the benefit for the HbAS goes on beyond 16 months, but it is unclear when that benefit would be lost. In any case reproductive capability should be around 120 months. Fourteen months of benefit compared to possibly 100 months of detriment does not sound like a profitable transaction.