You're of course entitled to your opinion or your own investment strategy and I agree, diverse holdings aren't a bad idea. But, might as well buy the real thing and diversify into other commodities like gold or silver for example.
Why would you give your money to an institution so that they can buy that BTC with your money and pay you a dividend, or not. Depending if you're betting on the right side of their ETF. Even if you do, half of the people will lose their money, the other half gets paid and the ETF issuer wins.