I look at it slightly differently. My thinking assumes a fixed rate mortgage on an affordable house, yada yada yada...
Your mortgage is stable and the interest rate is constant. Therefore your housing costs you less in real dollars as inflation and your income increase over time.
If the role of a bond is to add stability to your portfolio, I believe having a fixed rate (and tax-deductible) mortgage also has a stabilizing effect. Therefore, how much stability does one need, particularly during the acquisition phase of your life? I'd agree it's a lot lower than the folks who want to sell you bonds say it should be.
Another wrinkle is if you live in a HCOLA and your mortgage is by necessity rather large. Since so much is tied up in that, what's left and how should it be allocated? If one allocates too much to bonds, there's simply not enough left to allocate to stocks and stocks are what do the heavy lifting.
In retrospect 90% Stocks + 10% Bonds + Mortgage would produce better results and is probably less risky than it sounds. And that's my non-technical explanation of why I wish I'd been more "Aggressive" in my portfolio allocation.
Bonus: During an inflationary period such as were currently experiencing, holding a long, cheap mortgage is a superpower, IMO.