My guess is that on most random boards this will be the most useful token event. There are definitely boards in which the +1 card or +1 buy may be more valuable, but I feel this is much less common than the amazing utility of making a terminal action non-terminal.
I disagree. Lost Arts is only bonkers in the presence of terminal draw. Terminal draw is present in 50-70% of kingdoms, depending on how far you want to stretch the category (is Witch terminal draw? Is Watchtower?). In the remaining kingdoms, ~70% have action splitters. There probably exist some boards with villages but without terminal draw where it is worth it to get Lost Arts (Stables + Bridge, for example), but I think these are in the minority. While Lost Arts is usually a must-buy in the presence of terminal draw, this isn't always the case. I recently played an game with Lost Arts, Nobles, and Fishing Village. One of my opponents got Lost Arts on Nobles, but the rest of us got Fishing Villages and Nobles and skipped Lost Arts. Neither approach dominated the other.
On the other hand, Pathfinding is good on almost every board: it turns any cantrip+ into a Lab+. ~25% of all kingdom cards are cantrip+, so there is almost always at least one in the kingdom. Indeed, there is usually more than one cantrip worth buying; so it's more likely that you (and your opponent) can acquire more than 5 copies of your Pathfinding target. 8 coins is much harder to spike, but it is trivial to add cantrips to your deck while you build up to 8 coins. Then, you spike 8 midgame and immediately increase your deck's nonterminal draw by 5+ cards. There are some boards where Pathfinding might not be worth it: VP rushes and some slogs, but it is worth a purchase more often than Lost Arts, IMO.
I think Lost Arts is the better card, though, because of how game-warping it can be with cards like Ranger, Smithy, Margrave, and Torturer.