This game would be far more popular if every champ was designed by CertainlyT.
Not really,it would be basically Dota just without all the different actives on items to balance things out.
CertaintlyT champions are all "trap champions",the reason people like that is exactly the reason people hate them(well I'm mostly talking about kalista/yasuo/zed whereas thresh and darius have different,lesser issues),and if people would actually play to win it would create a situation where everyone suffers.
As to why everyone suffers: What would you pick if you were expecting a yasuo/kalista/zed pick?(note that they aren't limited to certaintlyT champs,fizz is another obvious example),basically something highly reliable either in defensive or offensive capabilities, annie,zilean,janna,kayle,ryze all come to mind,now there are two important things which these champions share:
1)Most people would consider them boring,and don't find playing them very enjoyable,the fact yasou/etc force people to play things they don't want to isn't very positive.
2)When properly countered by these champions the yasuo/kalista/zed players aren't having much fun either,there they just can't "cheat"(I like this term because skillshot reliability isn't balanced around their insane mobility, so the odds in such a matchup are heavily stacked in their favor) their way out of abilities,and in fact the binary nature of these champions isn't practicularly fun to play against as other things as well,but much better than playing as yasuo/etc vs them.
As it is they are fun because people enjoy hard countering matchups,they think to themselves "hurr durr I'm outplay lux she can't land a single thing on me she so bad lol im so gud",then when they get matched against annie/kayle "omfg fucking counter pickers why can't they pick something I can spam mobility to avoid all of their abilities".
So overall,They further invalidate champions who are already in a bad spot,they devolve gameplay by forcing highly reliable binary picks as other things simply do not work against them,people enjoy them because they enjoy rofl stomping people who pick off meta champions.
As to darius,ironically too binary,his kit doesn't have enough meaning he will just be some combination of a scary lane bully and jungler fodder,riot doesn't like champions who need to rely on being ganked to be handled,so he isn't allowed to be very strong.
Thresh is far too generalistic,has been overpowered since release,it's very hard to allow him to be strong without making him stronger than every other pick,which is mostly what he has been doing since release.