It's worth noting that in bot lane, you'll mostly be clearing sightstone wards.
That said, this is still ridiculous. Oracle used to be permanent and taking it was a gamble. Now it's almost useless - even if you don't die with it, the odds of finding enough wards to make it pay off are minimal. Your 75gp denied argument is bull****. That would be the case if you could clear the ward before they ever got any use out of it. However, the act of clearing it guarantees that all but the least map aware team to ever play the game will notice you in it, which means the ward has served its purpose.
So, let's look at it this way - your most heavily warded target lane, instead of returning 25 gold per ward kill, now returns only 10. You deny nothing in gold cost, since the wards are free from a core support item. You occasionally get a return when you find a non-sightstone ward placed by jungle, mid, or top(unless they've also purchased a sightstone, which I have seen), but you still deny only a fraction of the 75g cost, since the ward will have been up for sometime before you find it, and may have already spotted something of importance, and now also spots you. When this happens, you return 1/16th of the cost of the elixir, which is a whole lot more expensive now that you need to re-buy it ever 5 minutes.
There's also the problem of teemo(which is what happened in the game that prompted this post, wherein I discovered that oracle only lasted 5 minutes). His mushrooms return 0 gold when you kill them. However, he can continue to place them forever. To deal with him, you need to constantly expend 400g every 5 minutes, and get nothing for it(save whatever incidental wards you clear beyond the mushrooms), which basically makes the oracle a bigger drain then it's worth, which means the teemo wins whether you continue to buy oracles or not.
Yes, you can still rotate the burden of the oracle cost, but now, instead of whenever you die, it's whenever you die or every five minutes, whichever happens first.
Ultimately, this makes it a much larger financial burden on the team, with a much smaller return.
Edit: Qwid also makes a good point. Your whole argument was applicable before the nerf, and subject to chance. A gank might fail, a countergank might come in, you might not find enough wards to make the oracle pay off, but still not be able to gank because the lane is pushed(which is why you didn't find a ward), etc.
Sure, it's worth it if you make a successful kill, but that was true before the nerf as well. You just had more time to make it happen. Now, if it doesn't happen within 5 minutes, you just wasted 400g, and buying a second elixir as the jungler, while laning is still going on, is even more of a risk, since you're already down 400g from the last one. You can rotate it to the support, but then you'll get spotted in top and mid wards, and won't be able to clear them. Or you can buy it again, -800g, and maybe it will pay off this time, or maybe you will run into the same problems you did with the first one.
Something needs to be changed to offset the difference. Dropping the cost would be one way(what did Oracle's Extract cost? I never played the game modes where it was used, but I assume it was less then the elixir). Increasing the timer to a more reasonable 10 or 15 minutes would also help. That prevents it from being infinite, which I assume is why Riot changed it in the first place, but still rewards safe play and allows you more time to get some use out of it. A third method would be to ignore deaths, making it last for 5 minutes no matter what.