It is an investment in the same way a boat or an RV is an investment. The chances of making money off of them are slim to none (it can even be argued the same for a car) and they probably aren't going to be around in 50 years and if they are, probably of little value to anyone.
You're not investing to expect a monetary ROI but one in memories (as cheesy as that sounds). I don't want to imply that folks should go into DVC thinking that they can flip points for a profit. You (you plural) may save money in the same way that an RV may save you money on hotels if you travel across the country.
It is an investment in what it can potential save you in hotel cost at Disney for the next 50ish years. If it doesn't work for you (plural, not you personally), it doesn't mean there is something wrong with the system or something wrong with the individual's vacationing style, it's just one of the many systems of vacationing at Disney that are available.
I don't want to turn this into a DDP bashing, but we find that we don't use DDP often (only once or twice) because it doesn't mesh well with the way we eat (especially at Food and Wine). I don't begrudge those that use DDP.
Same for tickets, this year is the first time in 5 years that we didn't renew our annual passes. We looked at the times we would be vacationing and decided that 10 day tickets would be the way to go. We are taking our newly adopted son for the first time and the combination 10 day ticket plus water park is more economical for us than annual passes plus seperate tickets. I'm not shaking my fist at the annual passholders with their Tables in Wonderland card (well maybe a little,
).
We ran the numbers most likely as hard as you did, and found that purchasing at the time we did, it will work for us. We will probably never add on points through Disney, though. The price point we bought at 5 years ago was comfortable for us. Now...not so much.