I think you misunderstand the 70% number (or, at least, the one I commonly see on the net). It's not that 70% have failed, it's that 70% will fail. Give it time, and more RRoDs will appear, if that number is true. For me, when making a choice over which console to buy, that was unforgiveably shoddy console design by Microsoft.
What we can say for certain is that the RRoD % is very high already, and the 360 hasn't been on the market for very long. If reports of 33% of 360s failing are true, the end number will obviously be far higher. You may be happy to accept that, I'm not.
Finally, it's possible RRoD has been largely resolved now. However, it's also possible it hasn't. Only time will tell. Some will be happy to accept this risk, others won't.
You're ignoring a factor in your long-term prediction. While MS may or may not have addressed all issues that can contribute to the RROD, they have taken steps over time to attenuate the problem: an additional heatsink has been added, the CPU is now contributing less to the total heat of the system, and they have improved the conductivity points regarding the heatsink.
If you were to actually have a reliable way to graph this, you would see the failure rate declining while the total failure count declines as well.
Unless you have hard data that shows how effective or ineffective MS' steps to improve the situation have been, when those steps where taken, how many of the older units were still on shelves when they were taken, etc., then you have no way to reliably predict what the total failure percentage will be over the life of the product. Common sense, however, would tend to dictate that the total failure rate is dropping and has been dropping for some period of time unless every attempt to improve the heat-sink issue has had zero effect, and I seriously doubt that anyone posting on this site has data that demonstrates such a thing.