If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.
Your math is a bit off -- you're forgetting that your savings also grow at 4%/year while you're accumulating them. So if you save $2,000 / month and can get stable 4% return (after taxes), in 20 years you will have $612K.
The whole calculation, though, is based on guaranteed returns and if your returns are actually volatile (say, the mean is 4% with noticeable standard deviation), the situation changes.
And of course all these calculations are ignoring inflation.
If inflation is, say, 2%, then