I read the entire proposal and I wanted to say I appreciate the effort you put into this post.
However, I completely disagree that last hitting should be removed and that it is even a problem.
First, on the subject of last hitting being outdated: This isn't really true at all, this is a fundamental element in gameplay present in most games. That element is, if you take and action that results in a successful kill, you receive an immediate and direct reward for that action. Your gold zone proposal is more difficult to understand than the concept of last hitting. How do you indicate where zones begin or end on the map and communicate that to players? I've played over a thousand games of League of Legends and I probably couldn't tell you exactly where experience gains start and end in proximity to creeps. All the player is going to experience is a more rapid rise in ambient gold gain, which is essentially giving the player power while performing a far simpler set of actions in lane.
Second, you're essentially turning all laning into emphasizing pushing while removing all zoning with this proposal. Zoning is the concept of establishing an area of dominance with your champion that prevents the enemy champion from entering for fear of serious punishment/death. Harassment and zoning are what makes the laning phase fun and interesting for many people, including myself. Zoning can allow a champion, like Pantheon, to win a lane against a champion who has stronger pushing power because of his harass and killing potential.
Last hitting also involves decision making in the form of deciding how to use your champions abilities and auto attacks. Sometimes it can be simple, just kill creeps. But as the game goes on and the damage champions deal increases (and the damage from minions is less effective), those choices become more calculated, since the kill potential in lane increases.
Towers are another subject where I think you are missing the reasons why players choose whether or not to bring down a tower. Towers act as both a area of safety for players in lane as well as a tool for players to use to deny gold to their opponents. They also establish zones of control over the map by acting as safer retreat points should players venture into the jungle or towards the enemy base. By not taking a tower early, players aren't saying that towers aren't valuable enough. Rather, they are saying the ability to deny their opponent gold is more valuable than the global gold, experience, and map control they would receive. However, as the game progresses, denying gold in lane becomes less valuable than map control and global gold, so teams begin to take towers. Sometimes a team may want to take a very early tower, especially in bottom lane, so they will have greater map control over dragon and the bottom buffs.
Overall, I think Summoners Rift, as it currently is, benefits from having last hitting in it more than if it were removed.
The only problem, which honestly I personally don't have too much of an issue with, is the 0 cs support issue. There are a number of ways this could be addressed, but I think more ambient gold and more items that give gold is only a partially fixes it. Since there is very little game play around ambient gold, it's probably not the best solution.
What I think could potentially work is establishing a sort of assist system with minions, much in the same way champions are killed. If a champion landed hits on a minion but didn't result in the killing blow, they would receive a percentage bonus gold of that minions value.
Example: Janna and Ashe are in bottom lane. If Janna auto attacked each creep as it came down once, she'd get a small bonus when the minion was killed, maybe 2-3 gold per minion, while Ashe would continue to receive the full amount of gold she would normally get from a last hit (if she landed it). You could also make the minion-assist system scale, so if a non-last-hitting champion did more damage, they could get more gold. Using the example again, if Janna used a fully powered up tornado on the lane that brought the minions down to 30% hp, she'd get maybe 10-15 gold. Once again, if Ashe got the last hits, she'd still receive the full amount of gold. This would force bottom lanes to make decisions on how hard they wanted to push vs how much assist gold they wanted to maximize. Even though I think towers should still deny all gold if they score the last hit, I don't think assists where minions scored the final blow would be too problematic.