Quick note before Legitimate Dominion ContentTM: the art on this card reminds me a lot of Shadow of Mordor.
Bridge Troll vs. Bridge - The comparisons are natural, and the conclusion is too: Bridge > Bridge Troll. In kingdoms with both, unless you are really hurting for terminal space (Crossroads with no other options, just Necropolis), I think its clear that Bridge Troll doesn't stack up. The attack is annoying, but I feel like by the time you can consistently get a Bridge Troll into play, the -1 coin token just isn't enough of a hindrance to compete with the lower cost and the coin provided by Bridge. I suppose if you are ahead, the Bridge Troll attack can help you keep that advantage, but so does just trying to get further ahead by concentrating on improving your deck. Of course, Bridge Troll doesn't offer the same megaturn capabilities, which is a huge part of the power of Bridge.
Bridge Troll vs. Merchant Guild - I wouldn't have thought about this, so its interesting to compare the cards this way. I actually feel this is a good comparison, particularly when I think about when I want to buy/gain them. I often think about getting Merchant Guilds when my engine has starting running with the purpose of acquiring enough components quickly enough to be robust in the point-gathering phase. A single Bridge Troll will do more work than a Merchant Guild earlier, but it still is a terminal that doesn't draw. It does help you pick up multiple components over multiple turns, which means maybe you can get them a little earlier than Merchant Guilds.
In this case I think the comparison does actually get you a lot of the way there; many of the decks that want Bridge would still acquire it if it cost 5 or wasn't throneable. Certainly it is important that the card is not Bridge for the purposes of megaturns and the like. I suppose people who have a great intuitive feel for what Wharf (and Merchant Ship, etc) requires as far as villages go will be able to easily figure out how many villages can support how many Bridge Trolls at different points of the game; its pretty easy to figure out when you know you're reliably drawing your deck and getting into the rhythm of playing half of your Bridge Trolls every turn, but its non-trivial before that, especially considering the terminal space occupied by the draw required to get there (they can't all be Wharf). Personally, if I had a good enough feel for Bridge and Merchant Ship (duration terminal that doesn't draw), I feel like I could get 8/10 of the way there with Bridge Troll.