I agree, the engineering complexity of building a 50" tube would be cost prohibitive. Even Sony only went as big as 40" and I understand they had manufacturing issues there. One possibility may be to build the display in segments using smaller screens (Four 27" tubes should be sufficient).
Now the typical wall of TV's has a problem here due to physical separation of the tubes, resulting in gaps in the picture. What if the picture from each tube was carried by a bundle of fiber optic filaments up to a screen where the segments could be butted together. That could allow an uninterrupted 50" display. Now the geometry of the four individual pictures would have to be carefully adjusted to get everything to line up on the full image. If each tube could support 540p, then the full display would be 1080p.
Just a thought or should I be out selling the idea?