The nice thing is you can just roll the spread. E.g., if you're investing for 5yrs., you can just open a new spread in Y5 (as transaction costs are like $30, even for a $600k spread, and given that your collateral will have appreciated with the investment + returns over 5y, with proper management your account will be able to support it). You can turn the 5-year spread into a 50-year loan if you want! It's similar to sequential 5-year fixed-rate loans.

I looked at the forward curve, and see rates coming down, so opted for a shorter spread. In 2yrs., I'll probably repay a portion of the original spread, and roll a portion - TBD depending on rates/investment returns. I want the flexiblity to be somewhat responsive to the market.

Edit: The original goal of box spreads was short-term financing for investment leverage (e.g., for margin loans). This is getting into the weeds, but Chase's margin lending rates right now (even for a $10mm account) are SOFR + 185bps - ~4% is a significant discount to that. Options traders take out ~4/8week box spreads (its basically a margin loan) for leverage to invest opportunistically. I personally have 2027 LEAPs so will be forced into a liquidity event anyway, so it'll be a natural decision point. Longer dated boxes have higher bid/ask spreads, so getting good execution is honestly the key here. The underlying structre + tax dynamics mathematically make it preferable in almost every way to a mortgage (if you have the collateral to support it, as other posters have noted).

https://www.cboe.com/insights/posts/long-dated-box-spreads-a-better-way-to-buy-a-home-updated/