Anyone who has played League of Legends since Patch 8.11 when the marksman changes went live has seen a huge increase in the number of bruisers per game, largely because of their success in both the bottom and mid lanes. In fact, even in the professional scene teams have started to play without ADCs or mages and used compositions full of bruisers.
ADCs bitched until assassins and bruisers were irrelevant for nearly 3 years. ONE patch of ADCs not being auto-pick and the ADC community lashes out and goes "REEEEEEEEE LEEAGUE IS DEAD!!!"— Hashinshin (@Hashinshin) June 13, 2018
Bruisers are incredibly strong right now because they have the most item diversity and generally strong items available. For instance, consider the difference between the items for a bruiser and an ADC. ADCs only have offensive options, as even their defensive choices, such as Maw of Malmortius or Mercurial Scimitar, are still very offensive in nature.
Bruisers have the ability to get tanky while building damage. Items like Titanic Hydra give bruisers a huge advantage over other champions that must build either offensively or defensively, because Hydra gives more damage based on health, allowing a bruiser to be tanky and while still dealing damage.
We got our Mordekaiser/Shen bottom lane! One of the strongest 2v2's, 5 bruisers coming in hot for Snake! pic.twitter.com/bpWNcAhu3F— Rusty (@RustyLoL) June 15, 2018
Riot has no plans of changing this, as two of the new items to come out are Spear of Shojin and Atma's Reckoning. Spear of Shojin is a damage item that gives its user damage reduction when fighting multiple enemies and is clearly geared toward bruisers. Atma's Reckoning gives AD based on max health, much the way Titanic Hydra works, and is an excellent last item for health stacking champions.
The reason that bruisers are just now becoming staple picks for every lane is because of the marksman nerf and the acceleration of the game. After the marksman nerf, ADCs take roughly 30-35 minutes to hit their power spikes. There are a few exceptions, but in general that is too long, as games often end before that time.
Mages are viable as replacement options, but tend not to have the same sustained damage that bruisers bring to the table and therefore are not the first choice. Unless Riot Games plans to nerf bruiser items or focus more on ADCs, expect bruisers to continue their domination in League of Legends.