In your first paragraph your last two sentences complete contradict the rest of your paragraph. In the 1918 Spanish flu the second and third waves were actually more deadly than the first because the virus had slightly modified and hit a different population. It might actually be safer in September than it is in January. (As you said, we have no idea).
Since we may be facing a second wave at the heart of the fall, we are equally likely to have schools not in in-person settings in the spring as the fall.
Every assumption that says spring football being safer and more logistically more possible than fall football relies on an assumption that it is actually safer in the spring than the fall. Saying it will be safer in January than September is pure conjecture with absolutely no evidence to back it up. Diseases of this variety typically start upticks in the late fall more than the later summer (obviously this is a different virus)
If the second wave hits in December and the whole healthy fall was postponed until the spring only to get canceled for a second wave...that would mean there was some dumb assumptions being made.
True, I should have said, a spring season could make sense for a variety of reasons. It may not either.
That said, scientists can base some of the assumptions now on REAL data coming in now. Not models that have been wildly inaccurate literally from day to day, not just month to month. Research and testing can now be conducted based on what is actually happening with this illness, as opposed to what doctors think might happen in seven months with faulty models. That is powerful in research and will likely expedite things as more data rolls in and they can use stats to bolster research and protocols based on facts.
In addition, research, testing, facilities, trials, technology and knowledge are literally 100 years more advanced than when the Spanish flu ran its course. Of course, no one really knows, and there certainly could be (and maybe likely will be) a second wave. However, because of medical advancements, there's a chance there will be ways of dealing with a second wave in January that weren't available in June, July, or August. Maybe not, either. As you said, no one knows.
But the power of knowledge and data is what will figure this thing out. And, possibly buying time to continue to use that data as it pours in is important.