That's a deal breaker for me. Either offer interoperability or GFO. Otherwise this is just another vendor-lock in scheme to block people from moving away from a company's products.
MLS is mostly a co...
Which explains why it doesn't exist in Amethyst. Marmot is an upgrade from Signal to make it decentralized. Signal has a feature (perfect forward secrecy). The way they work around that is linking a device from the main account, and offering to copy historical messages to the linked device. If the feature you want doesn't exist in Signal, it probably will never exist in marmot. Of course don't quote me on anything. I am barely a spectator in this space. I have just been burned by Signal's centralization so I prefer marmot.