Is Treasure genuinely now playing second fiddle, or is this just groupthink, or hyperbole by a vocal minority?
It's a mixture of options 1 and 3, where the vocal minority mostly consists of people who are better at the game than a majority of people.
The important things to understand are how many stop cards your deck can support, how many it needs, how many it doesn't necessarily need but wants anyway, how time-consuming it is to increase the stop card space, how much time you have vs. what your opponent is doing, how fast the payload pays off vs. how costly it is to add it into your deck, how efficiently the payload spends your stop card space, and what's the best compromise between all of that.
Buying basic Treasures from the supply is the worst compromise that you'll ever go for. Obviously you won't ever go for something worse than what's always available. But they're really very inefficient at paying off fast, and they're also very inefficient at spending your stop card space, and a lot of the time, there's something better for the engine. If there isn't, that's one of the most common reasons to consider playing a non-engine strategy even if building an engine is technically possible.
If you can gain basic Treasures for "free" e.g. with Courtier, that's way faster than buying them from the supply, which makes them a lot more attractive as an engine payload option. It still has the problem of spending your stop card space rather inefficiently, but a lot of the time, mostly with trashing or strong draw available, you can accommodate that by building a bigger engine and it'll be worth it because the payload increases fast enough over time.
A lot of kingdom Treasures aren't as inefficient as the basic Treasures, either in terms of stop card efficiency or tempo efficiency, or both. Horn of Plenty and Fortune are both, for example, and they're also great payload options. Treasure Trove is better for tempo but worse for stop cards, so it's rarely ever seen as engine payload. Plunder is better for stop cards but arguably not better for tempo, but it's a very common payload. Fool's Gold is about on-par for stop cards, but better for tempo, and it's sometimes used for payload but not super often, which is also true about Hoard. Royal Seal and a bunch of others are neither, and they suck even more than the basic Treasures do.
A lot of kingdom Treasures also have utility besides just being payload. Coin of the Realm and Crown are anti-terminals, Loan and Counterfeit are trashers, IGG and Relic are attacks*, and Talisman, Quarry, Charm and Capital can help with early/mid game economy better than basic Treasures.
*IGG is not an attackBut yeah, TL;DR: buying Gold really is super bad for engines. Another way to put it is that a long-known fact is that on a board with Village and Smithy, Smithy BM is better than the engine, and another slightly more recently known fact is that on a board with Smithy and Navigator, Nagivator BM is better than Smithy BM so that gives you some perspective into how good basic Treasure only payload is for an engine.