I think people tend to over-estimate rather than under-estimate the rate at which the goodness of expenditures decline in the future. Among other things:

• Holden raises the possibility that giving opportunities will be worse in the future, but if you assign a 50% chance of giving opportunities being twice as good and a 50% chance of giving opportunities being half as good, then you should wait
• 7.9% is a pretty reasonable rate of return
• Debt can limit your options and flexibility for life decisions like further education, moving, and entrepreneurship
• If you have a fast-growing process which will eventually saturate then earlier donations are not as much better as they might seem without considering saturation
• Our technology is advancing, so we are getting closer to big decisions, opportunities, and problems involving those technologies, and spending on them will be easier when they are more in sight; there were cool opportunities for spending in the 1500s (funding science as it was just getting started, or sending colonists to the New World, who increased in numbers by something like a hundredfold since then), but many options were not even visible

However, independently of these issues it makes sense to give small amounts to "keep the habit" and stay in practice and aware of charity issues.