Standard procedure is to shift from the neck hold to another restraint ASAP as suffocation can and will result, especially with a suspect on their front. You only use the neckhold until you can apply other means. Even if someone is dying, if you take efforts to speed that death along, or doing things that can cause death isn't smart.
You're also not supposed to restrain suspects on the ground face down for a long period for the same reason. It can kill people, and it has killed people - you're supposed to prop them up against something - I doubt Floyd could have gotten his feet under him to try to stand if they'd sat him up , and it's relatively easy to press someone trying to get up like that back to the ground. He picked the worst possible way of restraining him, as a 'shoulder press' can quickly turn into a neck press and cut off circulation or the windpipe, if you're positioning yourself high up on the body.