Well if you're going to rule out people who have fallen into a pit of debt these days, where do you think they'll go when they're back on their feet? To your company that snubbed them?
The majority of the people who are on dinosaur computers are not, it's probably safe to say, people who are likely to show interest in your software even if they upgrade. It's also probably safe to say that if you win on features and quality, those who would be interested will buy it.
There's a cost/benefit to supporting old hardware, and very often the benefit is vastly outweighed by the cost. Econ 101. Sorry.
The target is "people who spend enough money to make writing code for them worthwhile," not "everyone ever."