This is far from trivial, but all of these have plenty of alternatives. Workshop used to be consistently undervalued by the community because they were all trained to go +buy when you could get a lot of the function of 2 from Wshop more effectively. Smoothing out coin can be done by having many options for cards (e.g. a rich engine environment with Hunting grounds, Smithy, Bazaar, and Wandering Minstrel), but returning cards to deck (e.g. Count, Courtyard), banking coin tokens, and a few other odds and ends. Scoring VP, well that is harder, but with some TfB, Goons, Monument, Opost, alt-VP, and a few other tricks this is also doable.
Obviously there are a few niche cards that will help you pick up components, gain Duchies, or take VP chips, but +Buy is a lot more common than those, and you can just consider those effectively "Virtual +Buy" for the sake of this discussion if it really helps you. The point I'm making is without +Buy (or these other things), in the absence of other payload engines are often just not viable. You either need +Buy / gaining to build an engine quickly enough to use it, or you need +Buy to have a payload of more than one Province a turn, for an engine to be viable. Many exceptions to this rule do exist, I know, but usually that involves a payload similar to "1 Province per turn + a strong Attack per turn" or the engine on the board is easier to build than anything else.
If you mean +action to be villages, sure, but villages are often overrated. If by +actions you mean anything with +1 or more actions on it and then ones that are effectively that (e.g. Tr, Prssn, Kc, Rc), then no.
Okay, you go ahead and try and build an engine with terminal draw and no Villages. Lemme know how that goes for you. I mean, ignoring Village Idiot scenarios where new players buy several Villages instead of real cards out of a pathological fear of any terminal collision ever, Villages are really important and their scarcity often is the sole determining factor in the viability of an engine.
Again look at the knights. Dame Molly is +2 actions and costs $5. Sir Marten is $4 with +2 buys. Of all the bonuses, why is +buy on the cheap Knight if not for balance?
Worker's village vs Bazaar suggest that +1 buy is worth less than $1 coin. Wv vs Lost City likewise suggests that 1 card is worth more than 1 buy.
I don't think you understand how cost works in Dominion at all, actually. Cost is only somewhat correlated with card strength. Cost in Dominion is saying how accessible the card should be, not how important it is. Cost dictates when you can buy a card - you can buy $2s at nearly any time, $4s you can only take one of in the first shuffle, $5s usually take a shuffle to obtain, etc. Cards with lower cost are easier to obtain multiple copies of, etc. For example, Chapel is one of the strongest cards in the game, but it costs $2. Why? Because it should be accessible to every player on either turn of their first shuffle, and because buying multiple Chapel doesn't make any sense. Does this mean heavy trashing is the worst strategy in the game, and is less important than looking at the top of your deck and removing Victory cards? Hell no.
Cards that give extra Buys are less important in the very early game and thus you're already dissuaded from purchasing them early. If you don't have an extra buy, they might as well cost everything in your hand, so cards you only need to buy early can cost nothing since their "real" cost is the use of your only Buy. And once you have them, it's only occasionally that you need to stack up a ton of extra Buys. This is why a lot of +Buy cards aren't expensive, not because they aren't important, but because when you need extra Buys you're spending all of your Buy phase resources on the card anyway, and because there's less benefit to spamming many +Buy cards than spamming +Cards cards.
Cards that give +Cards cost more, because you want a lot of them, especially in an engine which is more likely to have cards with +Buy, so the cost plays more of a factor in ensuring their scarcity.
+buy is great for engines, but it is not close to necessary to engines.
Engines need payload to be worth it. Ultimately, in many circumstances, engines take longer to set up than terminal draw Big Money takes to fire up, so you need some incentive to build an engine. The use of an Attack every turn plus VP gaining every turn is one way to do it. Gaining multiple Provinces a turn through +Buy is another. Being able to set up the engine quickly with gaining / +Buy can offset the speed advantage terminal draw BM has. In that sense, I've looked at many boards, seen a lack of +Buy / way to gain multiple cards a turn, and concluded "I can't get the engine to work" nearly as often as I've done the same due to a lack of Villages.
