No Stablecoins backed by Bitcoin are not mere tokens... There is a whole lending contract necessary to mint them, not just a token you mint and transact with RGB without need for observable chain. Now, if you take your DA challenge idea to its extreme you will find that it devolves to exactly always embedding blocks on Bitcoin... Because either there is no cost for the challenge in which case I will personally challenge everything just to prove a point, OR there is a high cost for challenging, which can't be refunded because you can never prove the data was withheld so you will just have to pay it. So if you want a chain to do things that require people to be aware of what is going on, like minting Stablecoins with Bitcoin lending, or with optimistic challengeable bridges etc... then you will need to keep posting the data all the time, except now you also need to publish the challenges, so more "spam". The other viable alternatives here are to use the same federation of the pegout as the DA committee AKA liquid... Or use merge mining AKA Rootstock (which is way harder to bootstrap). But if your blocks are meant to be small and infrequent, just embedding them on Bitcoin removes tons of complexity and ton of ways for things to go wrong. And again, if you are trying to do something difficult like exploring covenants or Simplicity... Choose your battles