I don't think killing a tower is tracked at all with kills or assist, same for objectives, dragon and baron.
Even so, you can be a big contribution to your team without getting a good kda. For example, say you are hard countered and/or their jungler is constantly in your lane. For the sake of example lets just say top lane. You know you can't go aggressive because the enemy laner will kill you, and if not the jungler that is always there surely will. So you play safe, don't get kills or assist and on the other side of the map your jungler keeps ganking bot feeding your adc immensely. At 20 minutes you haven't died once despite their jungler keeps trying to gank you. Mid laners are even~ish in kda and bot is 10 kills ahead and their team surrenders. Your kda is 0, mids is 1:1 and bot is 10:0. Is it fair that the people bot get more points even though you as a top laner spend tremendous effort not dying and keeping the focus on top so bot could keep winning?
No of course not, and this is just one example of kda not representing effort. Another could be a support having excellent ward coverage preventing ganks before they even happen. That also isn't something the game can take into account and many more situations can happen.
In the end its a team game, the whole team loses or wins.
While I agree that a lot of times this is true, because you cant garentee people will work with you, and more often then not they won't. But that is not to say it never works. I have had a lot of games were people did communicate strategies well and won because of it, a fair share of them by me starting.
I am not sure if I understand what you are saying correctly, but to me it looks like you are saying "if you do good you win" which is pretty much true for every game. Regardless of chosen strategy.
And yes most games that are won have the winning team at a higher kda. But you can get a win by first getting objective, thus gaining the lead and then getting kills. Or getting kills and using that to gain an advantage and thereby making it easier to take objectives. The end result of both is a higher kda the path to it however differs.