I agree it's complicated, but are we talking about more than 50 years?
That's not really a question I can answer, but I wouldn't be too surprised if it did.
Basically, to do it right, we'd need a lot of different fields that are in their infancy right now to mature more-or-less all at once. We're talking about taking macroscopic structural issues like brain volumes in different regions or the size of axon paths between them which are vaguely described by modern neuroscience and the huge number of genes potentially implicated in intelligence and then turning that into a single theory of what 'g' actually represents and then using that as a basis for therapy. And that will very likely depend on how good our ability to transfect large amounts of DNA into living cells are; right now you can't even move the gene for hemoglobin, much less whatever is needed for the hundreds of alleles you might have to change in a full overhaul. Throw in 15-20 years for FDA approval and your standard political controversy and you're looking at a big question mark.
I mean sure it could be that the problem isn't as difficult as it looks or the Singularity materializes and drops a solution in our laps, but I really wouldn't advise you to count on it.
Claimed dysgenic effects are very slow.
The problem is that the rate isn't constant; Vining, the guy people usually refer to for the "whites lose 1.6 IQ points a generations" figure, pointed out that the fertility differential is highly dependent on overall population growth. In periods of population increase the high IQ tend to lead the pack and the trend is eugenic, while in periods of population decline they trail and create a dysgenic trend.
Currently the US birth rate is below the death rate, with higher-IQ whites and east asians having the worst of it, so it wouldn't be surprising if by 2048 we see a much larger decrease in global IQs than you could have predicted with data from the 1940s birth cohort.
In "The Immorality of Having Children" (2013, pdf) Rachels presents the "Famine Relief Argument against Having Children":
They present this as a special case of Peter Singer's argument from Famine, Affluence, and Morality (1972), which is why they haven't called it something more reasonable like the "Opportunity Cost Argument".
[Note: the use of "Famine Relief" here is in reference to Peter Singer's 1972 example, but famine relief is not where your money does the most good. Treat the argument as "that money would be far better spent on GiveWell's top charities" or whatever organization you think is most effective.]
It's true that having and raising a child is very expensive. They use an estimate of $227k for the direct expenditure through age 18 while noting that college [1] and time costs could make this much higher. Let's use a higher estimate of $500k to account for these. Considered over twenty years, that's $25k/year or $2k/month. This puts it at the top of the range of expenses, next to housing. It's also true that this money can do a lot of good when spent on effective charities. At GiveWell's current best estimate of $2.3k this is enough money to save nearly one life per month. [2]
But perhaps we shouldn't be thinking of this money as an expense at all, and instead more as an investment? Could having kids be a contender for the most effective charity? That is, could having and raising kids be one of the most effective things you could do with your time and money?
For example you could convince your kid to be unusually generous, donating far more than they cost to raise. Except that it's much cheaper to convince other people's kids to be generous, and our influence on the adult behavior of our children is not that big. Alternatively, if you're unusually smart, by having kids you could help make there be more smart people in the future. But how many more generations will pass before we learn enough about the genetics of intelligence to make this aspect of parental genetics irrelevant? Rachels considers the idea that your having children might greatly benefit the world, and rightly finds it insufficient. While your child may do a lot of good, for the expense there are much better options. Having kids is not a contender for the most effective charity, or even very close.
Having kids is a special case of spending your time and money in ways that make you happy. A moral system for human beings needs to allow some amount of this. It's like working for $56k at a job you enjoy instead of getting $72k at a job you like less. [3] Or spending your free time reading instead of working extra hours building up a consulting business. Keeping in mind both the cost and that on average people don't seem to be happier parenting, if having kids is what would make you most happy for the expense in time and money then it seems justified.
(This is how Julia and I thought of it when deciding whether we should have kids.)
I also posted this on my blog.
[1] College is currently in a huge state of flux. Advertised costs are rising far faster than inflation as colleges realize they can get away with near perfect price discrimination in the form of "either pay the extremely high sticker price or give us all your financial data so we can determine exactly how much you can afford." At the same time online courses and mixed models are getting to where they can provide much of the value of traditional lecture courses, and in some ways do better. I have very little idea what to budget for college for a kid born now; likely costs range from "free" to "all you have".
[2] Rachels uses a much lower number:
Their Brenzel citation is to the Vaccine-Preventable Diseases section of the DCP2. The $205 number is "Estimated cost per death averted for the Traditional Immunization Program in Sub-Saharan Africa and South Asia" in table 20.5.
[3] This is a $16k difference, which comes from taking $500k over 20 years and dividing by two for the two parents, and then adding some for taxes. Though the earnings difference is likely to last more like 40 years.