Rumors are flying around that we'll see 2.1 on or around 1/13. If that truly is the case we'll be able to use all those apps in just over a month.
In answer to your question, 2.0 and 1.6 are both still fairly new, it's not like they've been around for years.
As a similar scenario, if I bought a computer a month or two ago that had Vista on it when I knew that 7 was around the corner I wouldn't complain too much. If that computer came with 2000 on it it would be a completely different ball game though.
If we use this analogy, then in the PC world, it would be like when Vista came out, then all new applications would only run on Vista and when Windows 7 comes out, then all new applications would only run on Windows 7. Sure, this does happen on the PC sometimes, but it does irritate us, but usually software developers write applications that support older versions of Windows for years.
With the fast pace of the Android OS releases with new features being added, developers want to write apps that use the new features and there's no way for an Android user to even update their phone-computer to support new features without rooting and installing a custom ROM (if it's even available). When you buy a PC, you can install the new version of Windows so that you can get access to newer apps that only run on newer Windows versions--and that's what we're missing with Android since it's not easy to update the Android OS ourselves. And that's why we expect the phone manufacturer/phone company to do this for us.
But as others have put it, pushing an Android update isn't just something the developers can just throw together and send out to everyone. It takes a lot of testing and bug fixing to end up with a quality product after doing the actual work of putting the new features in. I'll be happy with an update sometime early next year, but I will start getting impatient after Feb since I'm sure there will be 2.2, 2.3, or maybe even a 3.0 expected in the coming months of 2010 and I want to be able to install new apps on my phone-computer.