>For it to be useful long-term, you'll need to find a policy with both guaranteed renewal and guaranteed coverage increase as your income rises, before any chronic conditions appear (or are diagnosed) or things like car accidents occur and become pre-existing conditions.

That's only true if you want to increase your spending as your income increases. If you start out with an upper-middle-class paycheck and lifestyle and get disability insurance for that, and save 100% of any pay raises on top of that, then a simple cost of living adjustment is more than enough. And even without a COLA adjustment, your long-term savings should be accruing faster than inflation anyhow. Rough math says that if you can't save enough to replace 2% of your income in a year, then you can't afford to retire after working 50 years.

>That is, the total from all sources will be capped by the most generous policy at somewhere around 60% of pre-disability income.

This is more than it sounds like, since private disability benefits are untaxed. For anyone with a six figure income, that's $5k/mo in after-tax income. Plus, this income is 100% portable within the US - you no longer have to live within the high COL areas where the tech jobs are.