From a policy perspective, I think the idea was that encouraging people to invest in growth is worth the tax incentive. Over time, I'd say that has gone well (by satisfying that purpose), but it also justifies a reassessment of it now. Things are askew, as the bulk of the wealth subject to cap gains is held by the super-wealthy who could clearly afford to pay higher tax, and who will invest regardless based on the historic performance of equities.
The issue with implementing the change is that it absolutely will cause huge market upheaval if enacted on a cliff - if people know the rate is skyrocketing tomorrow, they will sell today. That will tank the market, which isn't good for the overall economy.
That could be managed by allowing people to "lock in" their built-in gain as of the date of the changeover, so that only prospective gains are subject to the higher tax. People that care about that issue would be able to afford to deal with the nuisance of tracking it. Another alternative would be a slow escalation - increase by 2%/year to the new level.