Discussing chains and chain care is dangerously close to religion for some riders, so expect a lot of strong opinions and few hard answers.
If you want to be level-headed about it, it becomes a question of relative expenditure. Frequent chain replacements will stretch the life of your cassette/freewheel and to a lesser extent chainrings. But you have to keep paying for the chains, and spend the time doing the maintenance. Eventually there's a break point where you end up paying money to save money.

If your method works, ie it lets you keep using the cassette, there's little to no benefit for you to look at the miles as well or instead of stretch.

As long as you have a cyclocomputer, tracking mileage is easy and definite. Measuring chain wear is not. Plenty of people here have reported replacing chains in what they thought was in time, only to be plagued by skipping and whatnot that wouldn't go away until they've replaced at least the cassette too.