Having done a little reading around it appears an older version of the game had an exploit that allowed players to drop their tower rating in order to get easier opponents in the colosseum; this has been since removed in an update much like the removal of healing gems.
Now, all that dropping your tower rating does is allow you to have a long run of fights before you have to worry about rising to the next challenge rating. You'd want to do this as the amount of silver awarded during events is based on chest type with no modifier for tower level. i.e. a legendary chest pays the same silver at level 12 or level 4. All that differs is the number of cards you get when you open it.
What you are seeing are a number of old accounts that haven't been touched in a while - they don't delete inactive accounts. I've migrated to tier 4 since my last post and have yet to draw anyone above 35k hp, with most being less than 15k with basic rainbow decks or with cards from earlier sets. I was expecting a tougher selection based on the lurkers in tier 5.