I also saw the Foos last week from a seated section. Ideally there would be sections which are all seated and those which are more flexi. I used to love being in the pit at gigs but at 45 with a weak bladder, those days are well past me. Doesn't mean I want to sit still though. The funny thing is that I could stand for hours without being uncomfortable but I wouldn't choose standing these days because I wouldn't be able to get in and out and back to my chosen vantage point. Also I've witnessed far too much aggro with folk pushing in and just can't be doing with that anymore. I'm also under 5 foot so unless I get early to get a spot right at the front (and I can't keep that due to menopausal bladder) then I'm not seeing a thing and definitely don't see the point of spending upwards of £100 to hear some music while staring at someone's back.
The seated section is tiered which means that if everyone stays seated, I get a great view and if everyone stands, I still get a great view. And I'm guaranteed that same view if I leave to go to the loo. I can see the problem though for those who can't stand at all, but also don't qualify for the disabled section so I, personally, would only stand if everyone else around is doing so, and the people behind don't mind. That did mean that I was seated for the whole Foos gig though when I was itching to get up. The folk in front did get up during The Pretender so I stood up too but I heard a clear "FFS" from behind so sat back down even though I was annoyed that my view was therefore blocked for that part of the gig. The standing people noticed though so sat down for the rest of it.
I noticed that standing or sitting seemed to happen organically in sections with everyone following each others lead. It seemed that the entire block next to us were on their feet the whole time whereas our block were sedate and seated. Still rubbish for any individuals who needed to be seated amongst the sea of standing folk though, which does show the need for a fully seated section or two. Also everyone in all blocks seemed to get up for the entirety of the encore.
I've been to gigs at murrayfield where the higher seating levels were strictly seated so it can be done although I guess this is for safety rather than comfort and the problem is that anyone with mobility issues probably wouldn't book the high, steep seats.
As for phones - I totally agree that a phone held overhead blocks the view even more and vasts seas of phones being held up is ridiculous. If people are able to take photos and videos from chest/eye level without taking up any more space then fair enough.
The only bad thing for me at the Foos were the teenagers in front (the ones who had briefly stood up) and the adults to my right who both clearly got bored during the songs they didn't know so were talking loudly and even playing back videos they had taken of previous songs at full volume. Totally rude and unnecessary.
Singing at a gig like Foos, Pink or TS is a given though and the sound of the crowd can give goosebumps. It's also clear that the artists feed off the energy of the crowd. Obviously not appropriate during theatre style concerts though, or for certain quieter, slower artists.
And don't get me started on singing along at the theatre. That's never ever appropriate.