You've said in the past that you don't like promos because if they're good enough to exist, you're depriving an expansion of a good card. Now that there are no more expansions forthcoming, does that change the equation? Specifically, there has been talk of a "Treasure Chest" expansion, but an objection is that such a product would only be really desirable for players who have all of the expansions. Why not create cards for such an expansion and just distribute them separately? You could make two new Seaside cards and sell/distribute them together as the Seaside mini-expansion, etc.
Well the main problem with Treasure Chest would be, that some other thing would be better. Even for the people who like Treasure Chest. Let's say Treasure Chest happens instead of Cornucopia. Is that better or worse? Well you don't get to look at Treasure Chest to make that call so it's unfair, but it gives a clearer picture than "what if we got Treasure Chest instead of nothing at all."
It's true that requiring you to have bought all the expansions would be a downside, but that's not really the case except for the tokens and Potions, and I could avoid using tokens, the Prosperity cards could just be a treasure and something that costs $7. And I could pass up doing a new Prize, or I could make another card that uses Prizes. So, the best case scenario is, there's one card that you can't use if you didn't buy Alchemy or Base Cards, and the rest of it works fine for anyone. The rulebook covers duration cards etc.
You can also argue that probably any 9th expansion is only for people who bought 8 expansions (or maybe 6-7 leaving out 1-2 that sounded bad to them), no matter what the contents.
The big advantage of Treasure Chest is that it would be easier to make than a new small set. That is how it is a topic at all.
Promos aren't sold, they are promotional, so, not to be greedy but realistic, we would not just make 13 promos to give away if they could be an expansion instead. I don't know how small you can get and still have a product; I bet it's not great going smaller than 13.
Your basic premise that a lack of expansions means promos aren't missing out on expansions is fine though.