Exchange-traded funds reached several milestones recently. At the end of last month, they turned 20 and their global assets, along with exchange-traded products, breached the $2 trillion mark with a collective $2.05 trillion.

Many people now consider ETFs to be a mature financial instrument, suggesting future declines in product design and innovation, number and type of users, uses, trading volumes and product launches, exchanges listing them, indices, markets and asset classes covered and brokers trading them.