Thank you. Yeah, shooting a night game would be a whole different deal. At one point during the day game I was shooting, it got cloudy and we had some snow flurries and I had to bump the ISO up to 1250 to get my frame rate fast enough. Could certainly see 6400 being a minimum ISO at night.
I really like the 100-400L. I bought it earlier this year right after the mkII came out and they were selling off the last of the mkIs at a good discount. I originally bought it as a wildlife lens, but it worked really well for football as well. The one drawback I found when shooting the football game was that I kept pulling back on the zoom too much so I could comfortably fit the action in the frame. Once I got home, I realized that I should have been out closer to 400mm for a lot more of the photos I took so I wouldn't have had to crop as much. That would have been a benefit of having a prime of that range, forcing you to stay tight. On the other hand, the 100-400 made it a lot easier to find the action and zoom in (if you remember before the play is over).
The push/pull design is good and bad. It's good because changing the zoom is really quick by just extending your arm. The drawback is that the zoom lock ring can be a little bit of a hassle to loosen and tighten when you are moving around a lot (especially if you're wearing gloves in a colder environment).
From a technical sense, it does vignette a good amount wide open on FF, but that's easy to correct in post. The IS system is older, so it makes more noise than some other lenses, but it's not objectionable even in quieter environments like churches. I can pretty consistently get 2-2.5 stops of stabilization out of it. Sharpness isn't quite as good as my 70-200 f4, but it's still pretty crisp (as a reference, there were only 3-4 photos out of the ~100 picks I had from this football game that I bothered to boost the sharpness in Lightroom). Color reproduction is very nice.
Overall, I think I'll hold onto the 100-400 for quite some time. It's just a really useful zoom range to cover.