However, what are the chances that there might not be a 2020 season for the NFL, also the NBA and others?
For me the possibility of no 2020 NFL season is next to zero. It would take very special circumstances to cancel it all together, there is just too much money involved and as we all know money makes the world go round

Look at german football (soccer) Bundesliga. Lots of shops were still closed, gatherings of more than 3 persons were forbidden, masks mandatory in all indoor places, but football started again. And the Bundesliga isn't as powerful as the NFL is. Now you basically have all leagues in Europe playing again, that weren't cancelled; Basketball is played again too. All this with sophisticated hygiene concepts, no spectators and it worked; I didn't hear of any issues.
This virus is relatively easy to control in the end, if everyone is on the same page and shows just some discipline (a bit of social distancing and wearing a mask is enough). What a lot of people in the US don't seem to understand is, that if you show discipline for some weeks, you can be "done" with this relatively fast and go back to normal step by step. If you refuse to co-operate and insist on your "freedom" (which is good in general, but not with this matter), you will be stuck in this situation waaaaaaaaaaay longer.
The situation in general is embarrassing enough for the US and if they fail to have something like a NFL season, this would be even more embarrassing imo, because it's all in their power to improve the situation.
Even if the situation is like it is now by August/September, I still think the NFL would start (because money). Only if the situation worsens even more, I could think of the season being postponed. Only postponed though, because money again. To cancel the whole season, I don't know what would have to happen. Since you are also active in the US election thread, I will post this quote from Condoleezza Rice here, which I also posted/referred to in that thread:
Weve had elections in this country when we were at war, even when we were in [a] civil war. And we should have the elections on time.
Source: https://www.snopes.com/fact-check/presidential-election-postponed/-
As for NBA and MLB, this is even easier to manage imo. NBA has small rosters/staff and Baseball is no-contact sport 99% of the time and outdoor in a lot of places; biggest problem is probably the chewing tobacco and the spitting

For NHL not sure, because they have bigger rosters, there is a lot of contact and the arenas are cold-ish, which is something this virus seems to prefer from all we know.