Back in the day items were tied to a level requirement. When you equipped level 30 items you could only match up with other level 30s. Same for level 28, 25 and so on.
perhaps with a little logic X amount of mythicals could block dropping down past rank Y.
OR! Maybe the funky points assignment used for war could be used. If your build has a 5k war ranking you would automatically be pushed up to rank 7 and could not leave r7 regardless of losses. .
would take some adjustments but better than nothing?