Trundle is not weak because of tanks being weak, he's weak for these reasons...
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He was nerfed quite hard after seeing some pro play. They hit both his base damage and utility, which essentially cripples him for the average player.
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He's all sustained damage with zero burst. The meta highly favors burst damage and quick trades, which are both his weakest areas.
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Defensive itemization and scaling are both weak. He depends on defensive itemization being strong more than tanks do, as health and cdr are his best stats, coupled with him being squishy pre-ult and without items.
He would've fallen out of favor, once the season ended, even without the nerfs. However, Riot rarely thinks in the long term when balancing. All he needs are the changes to his E reverted and he'll be in a better spot, but still underplayed.