We're like a bunch of rabid speculators in the stock market... heh. Quite funny. That said, I'm going to speculate on this as well.
18 Months... Not a chance. As all of you before me have said, theres not a chance if that ever comming about. I'd only add that for it to come about would require drastic changes and implementations on both the Studio's and the hardware manufacturers sides. IE: Studios completely phasing out production of DVD's and Manufacturers ceasing production of DVD players. And that is simply not going to happen, as its fairly easy to see that Studios haven't even ceased productions of VHS and Manufacturers haven't stopped making VHS and DVD/VHS combo players.
Even with prices dropping significantly, lets say the arbitrary number of $10-15 a disc and $100 a player as someone before me mentioned. Consumers wont necessarily be jumping ship. The issue and problems Blu-ray faces is not mainly a cost issue. The primary issue, at least in my mind, is complex and revolves around the concepts of Performance, Understanding, and Value. Cost is secondary to this, in my opinion, because I believe it is fairly well established that people will spend more, gladly, if they believe what they are buying is superior to what they had before.
What the common consumer lacks is simply the understanding of what HD is, and how it works. I'm often amused as I talk to my friends, and head over to their places and they show me what they have. More often then not they have their systems setup incorrectly, and this is simply due to lack of information on their part. I was in a cdplus store yesterday and ran into a friend of mine who was talking with his girlfriend (who worked there), and I was looking through their Blu-ray selection when she started talking about how they seem to be getting rid of HD-DVD's. I told her that is because HD-DVD is no longer being supported by its creators, and she immediately said that she knew all along that Blu-ray was better then HD-DVD. I just laughed to myself and held my tongue. Another example of having no clue comes from my old man. I recently traveled down to visit him and he was showing off his brand new 32" Sharp LCD panel. He was all smiles as he was extolling the virtues of his $3000 purchase, and its ability to display 1080p. He also finally decided to get HD Satellite (which was a huge upgrade for him since previous to that he was just picking up local channels via rabbit ears). I did a quick once over of his quaint system and noticed immediately that the satellite receiver was connected via component cable and not HDMI. So I quickly checked the receivers settings to see if it had been configured to output at 720p at least, and it wasn't. So not only was my old man clueless to the fact he wasn't watching 1080p HD tv, the satellite installer didn't even know enough to set the receiver to output correctly for the chosen wires. Further, my old man is still using a VHS player, and his entire collection is comrprised of simply VHS tapes.
So all of that said, I believe even still that Blu-ray is here to stay. It has found a market, and it will continue to grow. However, I don't believe it will be all that long before we start seeing reliable and quick methods for streaming HD content via internet with all the same goodies as Blu-ray. This is the real future for HD, and movies/tv.