Each team has a 50% chance at winning, however that doesn't include a lot of factors: MMR inaccuracy, tilt, champion selection, team comp, and simply how well someone is playing that day.
Get that all out of the way, and you're probably not that close to 50%.
Saying that the team who is winning early will eventually win the game 75% of the time isn't unreasonable... I mean, let's break it down (completely made up):
Let's say 40% of games are really close throughout, 40% are stomps, and 20% are attempted comebacks with a better scaling team.
Of the first 40%, the team with first tower gets a bit of an advantage and wins say 60% of the time (a reasonable reward for good early play). That results in a 24-16 splits on the group.
Of the next 40% the games are stomps with basically 100% won after first tower. These are games where a team simply picked a better comp, played better throughout, or otherwise dominated (maybe someone fed hard, etc...).
Of the final 20%, the first tower doesn't mean hardly anything, give them a 50% chance at winning. The late game comp makes up the gold disadvantage (if the late game comp were better players, they'd be winning in the stomp group).
Then your total would end up at 74% to 26%.
So I'm not really concerned about that.
Give me a stat that says one tower falls but the game is pretty close overall (either another tower is taken in response shortly after, or the other team is within 300g after the tower) and the win rate is still over 65%, then we can see a problem.
Saying that a good number of games the team is already winning hard and first tower is just a necessary step to victory doesn't mean much.