The modding community doesn't do anything for the success of mass market phones...sure, they might help sell a few thousand more to those of us that read forums like this, but they also potentially cost the carrier and manufacturers a lot of headaches, from bricked phones to "stealing" services to fielding customer service calls from clueless second-owners of a modded phone.
And the thing that makes modding great for us, is BAD for the manufacturer. Why buy the latest next-gen phone when you already have a modded last-gen phone that does just about as well?
I own a Milestone. I love it. I wish I could do the things the Droid owners can do, but I still bought it knowing it had a locked bootloader, because ultimately, even without custom ROMS, it's still one hell of a flexible device, even running 2.1 (and 2.01 for that matter, which it had when the Droidboyz were already sporting 2.1 cooked ROMS)
When the GSM Droid X variant comes out, I'll seriously consider buying it too. For the same reason, it's a great phone even locked down, and Motorola doesn't skimp on hardware (I'm talking to YOU, Samsung and HTC).