Hayes is guarenteed jack shyt, so any big that comes it will automatically be competing for the backup center slot, which will become the starting center slot the moment AD gets hurt and starts missing games, at which point both "extra bigs" will need heavy minutes.
Not to mention that AD will want to play 4 as often as he can, which means even more minutes for those bigs.
Remember how critical it was to have both Dwight and JaVale for that 2020 title season? And that was a year AD was healthy all season. The Lakers NEED one more playable big, there is zero doubt about that.