Meh. They're both the same cost and they're both trashers, but I feel they're easily different enough to defend the existence of Trade Route. Forager isn't even close to strictly superior.
I think Forager is as close to strictly superior as you can get without being so in comparison to Trade Route. That +Action alone makes it very compelling, and the +$
it pretty much automatically provides is much stronger than the late +$
or +$
Trade Route will often provide, especially since you will likely not have as many targets by then.
First of all, Forager gets stronger by trashing, while Trade Route gets stronger by buying. You might green just a bit earlier than you usually would if you want to power up your Trade Routes, but at least you're not forced to destroy a 6-cost card (Gold) if you want to make it strong.
I'm not so sure destroying Gold for it's own sake earlier on is the best use for Forager. I think the time you would want to do that is in a double Tactician board in an immediate example or very specific cases where you want to squeeze the extra dollar out of your deck in the last turn. You also may be hurting yourself for the benefit of money with a couple of trashes by greening early with Trade Route, especially if it's the only method of trashing in the game!
Second of all, they're both kingdom-dependent: Trade Route gets stronger with alt-VP, especially functional alt-VP like Great Hall and Nobles, while Forager likes alt-treasure, especially early-game stuff (Quarry, Talisman, Loan) or stuff you might want to trash anyway (IGG). Those are pretty different kinds of boards.
Trade Route might be worth pursuing in a deck with Nobles and Great Hall. In a game where that card is the only +buy, it would be worth it for sure, but the fact that it is a terminal really hurts it early on, making you want two of them at most a lot of the time. Forager doesn't have that problem, and you can buy 3 or 4 Foragers right off the bat and be fine.
You also want to be careful with alt-treasure trashing and Forager. It is true that you gain benefits, but so does your opponent, taking into account what best helps your deck move along. Trashing IGG can be a good choice, trashing Quarry can likely be bad, especially with Forager buys in the first place to increase your gains. I'm not entirely sure if it's worth trashing as many treasures as you have to boost Forager or not. I don't think it is, but I'm not really sure about it.
In general, I agree Forager is stronger, but they're not as similar as they might seem.
The thing to take away from this is that Trade Route is an 'endgame' trasher, and not the great kind like Butcher or Graverobber. As previous people have said, every kingdom is it's own strategy. One game where Trade Route is awful and another where it is great. I find the best times Trade Route shines is when there are no other trashers on the board but a gargantuan engine besides.