You can die by boredom, thats what I beleive. If you have reached wave 1,000,000,000 in every map in survival you should think about doing something fun, for example some of these things or special modes.
Agency-Use only pistols and flashbangs
Die Rich-Try to save up as much money as possible without spending it
One On One-Try and kill your teamate with them trying to kill you.
Impossible- Dont use any guns excpet for the first 5 waves
Enemy is my resource- Use only the weapons dropped by Enemies with no bullot refill
LongShot-Use only snipers and Frags
If you have any more interesting stuff leave some in the comments section, have fun or don't play, thats what I think!