Should considerations be made if half of a project group drops the course?
I've seen this happen before. What ended up happening was that Mr. Doe ended up being allowed to do an individual assignment on par with the level of effort as a member the group assignment.

Losing members does not change deadlines. The same result is expected, i.e. a paper is due on a deadline. That shouldn't change, even though your group has changed. Half a group doesn't mean you can get twice as long to work on it, mostly because deadlines rarely move as a result of group changes, and practically because the schedule won't account for it.

What has changed is the amount of work you can realistically accomplish. Instead of having four person-units to work with, you have two person-units to work with. You will not be able to make the same effort as four people, but I highly doubt the professor will expect you to be able to meet that level of effort, simply because asking a human to fill in as two humans rarely ever works in the real world, at least not for long before that human breaks down.

Do not expect to change your deadline. Expect to change how much work you will complete by that deadline. Talk with your professor about how much effort the two of you will need to apply to meet his expectations, as opposed to whether or not you will meet his deadlines. Does he expect you to perform as much work as four students will do, or just meet the same timeline?

If he also wants you to do twice as much work as other students, then I'd have issue, but if he only wants you to meet deadlines, and will take into account that you're short 50% of a normal group, then I wouldn't find any real merit to saying the deadline is unreasonable.