Comparing leverage on ETF to be able to borrow billions of dollars at 0% with no margin call is like comparing an elephant to a mosquito.
What is exactly the advantage for the investor? These billions are only invested in BTC.
The fact that there is no margin call, no risk of liquidation (or if you prefer a risk so low that is extremely unlikely, especially compared to a leveraged position in an ETF, that can be margin called).
One of the main sources in the Microstrategy business is the convertible bond trading community.
Michael Saylor has been riding the volatility of his stock to sell a lot of volatility to the hedge fund community, which bought the cheap convertible bonds, selling the shares (or better, selling bitcoin) and gaining when volatility in MSTR price goes up (and the multiplier grows).
This is why Michael Saylor has been quite profitable in selling very low-priced debt and keeping his debt costs low. Selling volatility to institutional investors, operating in a delta neutral way.
Given the success that MSTR has had with this product, it is not incompatible for me that demand will continue to grow even if XXI goes on to offer a similar product, as it offers security, which is what a bondholder is looking for, as well as the highest yield in the entire bond market, which is why demand has not stopped increasing.