Since everyone seems to be weighing in on this, I figured I may as well too. I'm one of those people who generally likes to get the latest and greatest but you need to take all factors into consideration. First, I'm generally happy with my 1520. Sure, it would be nice to have something better, but not if they are planning on incorporating specific hardware that requires Windows 10 to be taken advantage of or even work at all. I would hate to get a new phone now just to have a slightly faster processor or whatever and then 6 months from now have the hassle of trying to sell my phone and pay it off and then get another.
Secondly, Microsoft controls only around 3% of the smartphone market. Of these, the vast majority are low-end phone users. These users use WP because it meets their requirements in both functionality and price. They don't care so much about the specs of the phone. We high-end users are very few. As many people have pointed out, a lot of high-end users like to upgrade every year. Chances are they are already using Microsoft apps and services and will continue to do so even if they switch to another OS. Then after another year they will look at the Windows phones again and chances are they might miss the live tiles or another aspect and then switch back. If they choose to stay with iPhone or Android, it is a customer lost, at least as far as hardware is concerned, however, there are very few people like that. Rather than to go through the hassle of getting a temporary phone approved from the carriers, Microsoft has chosen to keep up with the low-end phones for people who only care about price and not specs until they are ready with the major upgrade. It's a calculated risk.
Lastly, once everything is ready, this will give Microsoft an opportunity to show that they have created something that is significantly better in all aspects and give others a compelling reason to switch. Whether this can be accomplished or not remains to be seen but I highly doubt trying to salvage a very small percentage of current users by offering a minimal upgrade and then upgrade those phones to Windows 10 and not be able to take advantage of features like Continuum and Windows Hello, etc. is going to help much.