Yep. I had never looked into the details of WAIR calculation, so I'm a little surprised also.
I agree with you. When we think about our current portfolio, we should be thinking about parameters of our current portfolio, not all of history.
ANAR has this same issue. It is a calculation over all history, including all your paid off, charged off, etc loans.
In LC's defense, I would say that WAIR and ANAR are both calculated over all history, so if you were to compare ANAR with something and want a reasonable comparison, then a weighted average interest rate that is averaged over the same history is the right one. In other words LC's ANAR and WAIR are somewhat compatible with one another.
I prefer a return estimate that is more current than ANAR. I used to have an ANAR of 10%+, and now I have 8%+, so people tend to think "He's earning 8%+ now" but actually I'm only earning 6%+ right now, which averaged with the old 10%+ gives me 8%+.
When they thought up these measures, I don't think they were thinking about how portfolios would look 10 years later!
For myself, I compute IRR over various time periods.