It is not reasonable. Difficulty will continue to rise.
You don't know that. Just a couple weeks ago you were just as sure that difficulty would continue to rise by 50% every 2016 blocks. Last rise was 13% I believe, next will likely be the same or potentially lower.
Similarly on the other side of the fence, the ROI is unfortunately unknown, that's the problem with this thread. ROI now is good, in the future it will be...? I have already made roughly 400% ROI from my cluster, including all costs to date, in a fraction of a year, but the rate of return is decreasing due to various factors. What I end up with by the end of the year I care not to calculate because it will be meaningless speculation. Regardless, mining has been good to me as an investment. But the strategy and outlook I take into the future will have to be far different than the one I used in the past.
So, ROI, we'll see. Nothing is guaranteed. Same with stocks.