Still, it is possible and I don't think any way to get infinite VP should exist in this game
There's a lot of possible infinite loops that get infinite VPs, most of them almost impossible to happen in a random game. See this topic:
http://forum.dominionstrategy.com/index.php?topic=20320.0
If we're being pedantic, it's impossible to get infinite VP because Dominion takes place in finite time. Eventually, the players die. Hopefully
before that, they get bored and end the loop. Even more likely, the other player sees that they will lose and forfeit.
It seems the real issue here is, should there be a strategy that, for any N, allows a player to reach more than N victory points? As Carline indicated, already these strategies exist. But this strategy can exist without any combos. Simply adding Monument to the game means players can reach any victory point they desire. Your opponent may seek to end the game. Or, they see that they will lose and forfeit. Or, they have a strategy to get more points quickly, even though there strategy is bounded in the number of points they can get.
All that being said,
I think the smaller text, infinite-loop version of Canary is better. I don't know if the discrepancy of upvotes the two posts get is an indication of other people's thoughts. If you really must avoid infinite loops, you can try this:
$1
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When you trash this, you may replay a face-up Action card you played once this turn that's still in play.
once you replay it, is no longer "played once." All infinite loops with Canary are thus avoided. This then means you can't throne room a card, trash a Canary to then play that card again. Is this nerf worth avoiding the infinite loop? Hard to say.
I have to root for Canary as I had the same idea (although mine [haha] allowed you to discard the Canary instead of trashing it, and it costed less as to not upgrade to gold. I think your version that stays trashed and upgrades to gold is better).