You do NOT assume the income will always be at this level
For what it's worth, that was a common discussion here at least seven years ago. There were Adsense publishers that had the rug pulled from under them when their CPC rates dropped. Others making 10k a month through one or more niche sites/blogs, only to have their account disabled for whatever reason. Anyone who has taken the time to read up on Adsense should know that you never gamble your livelihood on it.