Prior to the Bush era tax cuts, all dividends were taxed at the income rate, which I think explains the biggest motivation in the shift from dividends to buybacks in that era.

Nowadays, as you say, there is no difference in tax rates for qualified dividends. However, one big remaining benefit of buybacks is that a dividend forces one to incur a taxable event when the dividend is issued, even if one chooses to immediately re-invest the dividend in the company (as many people still in the accumulation phase of investing do). On the other hand, a buyback does not force those people to sell.

On the other hand, a buyback should lower the market cap of a stock, so cap-weighted index funds ought to sell and re-balance when a buyback is issued, so most index investors would seemingly end up selling. However, they end up being able to avoid most of the capital gains taxes by re-balancing through redemptions and heartbeat trades.