While I never tried the 400DO, I used to have a 70-300DO which had it's share of, let's call it "character"! Have they got better in their design and usage of DO elements?
Why can't Canon make an affordable supertelephoto lens?
The biggest problem is big glass will generally mean big money. SLR AF lenses will be generally limited to f/5.6 (stretched to f/6.3 on 3rd party lenses) to maintain AF ability. As the focal length goes up, the physical aperture size goes up. With photographic lenses, it seems going much above 100mm aperture starts getting rather expensive. The Sigma 120-300 f/2.8 is probably the most affordable lens in that class, as anything above it starts to get really expensive quick.
For a glimmer of hope, I guess we could look at the world of astronomical telescopes. Skywatcher do a StarTravel 120, which as its name implies has an aperture of 120mm. The configuration of this particular scope is 600mm focal length at f/5. It's around £250, which is in the ball park of, say the EF 85mm f/1.8. Why is it so cheap for the size? There's only two optical elements in it, forming an achromatic doublet. In short, it isn't well corrected for colour aberration, and only really designed to work at one focus distance: infinity. You need to do a lot to it to get up to the quality expected of even cheap photographic lenses, and the costs will add up quickly.
A budget supertele is possible I guess, but given that up to 500mm is already quite affordable, the market for a budget lens much longer than that rather niche.
