He has pretty bad base stats for a tank, and his only direct tanking ability is his ult due to the bonus HP, and guess what. That amount of HP will normally allow him to survive like one extra AA from the ADC. Aside from his ult all he has is his passive to sustain him.
His utility is kinda poopy, all he has is wither and that just gets cleansed via QSS/Crucible, and then he gets kited to oblivion and can't really do anything in team fights.
If he can't split push he struggles to be relevant, so how is that not a good example of falling off?