There is little evidence that wealthy people actually borrow for income in any significant way. For example, this paper[0] finds that borrowing only accounts for 1-2% of economic income among the top 1%.

This makes sense. Borrowing for income in most scenarios is strictly worse financially than recognizing conventional income if you actually do the math. Wealthy people are optimizing for financial outcomes, not avoiding taxes per se.

[0] https://www.sciencedirect.com/science/article/abs/pii/S00472...