@Riot, the actual reason why deadmans plate is used more than sunfire or randuins omen.

most champions that need an armor/health item are tanks or bruisers, some other probably do, but most are in that category, and deadmans gives the best utility to them.

sunfire is only effective on characters that have low wave clear and are not jungling or support, and it is also only good when got early, which limits its use considerably, so if you have numbers that suggest that sunfire is being bought almost never, those numbers are correct. sunfire is a very specialized item, and it should probably remain that way.

Randuins is a very good item, but for it to work in the correct fashion you either need to be a duelist fighting an auto attacker, or you need to already have an effective gapcloser to make the slow worthwhile in fights.

the reason why deadmans is better than these items and is used more often is because it has a more general use, movespeed is good in almost any situation, especially for characters that inherently lack the ability to engage with abilities effectively. the slow on auto and extra damage are also very effective when it comes assisting the characters that this item was designed for.

Now that I mention who the item is designed for and I'm already sure you understand this, the item is primarily designed and used by immobile fighters and tanks that particularly need the mobility. most tanks that are still currently in meta or are even considered playable are jungling tanks, which means that their desires of having waveclear is already covered by cinderhulk, which in turn means that their choice is realistically between randuins and deadmans. randuins is designed as an anti carry tool, but the carry needs to be autoattacking the tank for it to fulfill its purpose, and against any relatively good player they would know not to autoattack the tank, and tanks also generally have enough crowd control to make it so the active is practically useless in many situations, especially the ones that have abilities that close distance, which in turn means that randuins turns into a defensive option that is built reactively instead of actively. the types of fighters that are currently seen in the meta are either the type that don't get a whole bunch of defenses, and if they do they prefer the extra damage of deadmans, so this nerf will not effect them, and the type of fighters that don't have a bunch of mobility, so this nerf will not effect them either, because deadmans is the only option that shores up the mobility issues they have.

how to fix this seeming problem. its actually pretty obvious if you ask me, if you want champions to use sunfire, buff up some champions that historically went top lane but make sure they don't get any good waveclear, shen would be a good place to start, if you buff shen so he can go top as an effective meta pick, you will start to see sunfire more, if you want to see randuins more, your probably going to need to buff the item and have the attack speed slow as part of its aoe, or give it movespeed, or tenacity, something to make this item an attractive pick for the big lumbering giants that the tank and juggernaut roles are known for.

nerfing deadmans in any way that doesnt completely destroy the item will not make the item any less prevelant.