Mistress Death hasn't been eliminated. She simply is hidden from King Thanos. If she were totally gone, none of the vast multitudes of heroes, villains, cosmic beings and abstracts. For more you can visit my spoiler-filled post of Thanos #14 at CBR:
King Thanos is immensely powerful, that is for certain. From fighting and killing all those heroes, villains and cosmic abstracts/beings to casually one-shotting Thanos as if he were nothing. His power is among the highest we've ever seen.
As for Hulk/Maestro, I suspect King Thanos is utilizing him as his own personal WMD to annihilate any hordes of powerful enemies that may try to lay siege to his fortress.
You may be right about Lady Death or she may have gone or perished after the battles with the others. There is definitely a horde of something headed into next issue. I do have to wonder why K.T. would fear anything if death isn't around. Quick thought I wonder if the horde is the Fulcrum from the last Eternals series? Whatever the reason for keeping the Maestro alive is sure to back fire. The thing about WMDs is they can annihilate user as well as the target and Maestro is a very smart WMD with an ego to match.