Again, by your logic no one would ever make a high budget launch game. But they do, regularly. AAA console launch games tend to have very high tie ratios, and relatively long shelf lives, since new console buyers continue to buy them over a relatively long period. Plus you're investing a long-term franchise user base.
Obviously, if you can down-port relatively easily and without affecting the quality of your lead platforms, it makes sense to do so. That has not been the case in previous gen cross-overs. It may be somewhat easier this time, so we may see more cross-gen releases than in previous transitions.
Also you're making a lot of arbitrary assumptions about the game's budget. Budget is not a linear function relative to development time.