TL;DR
Nothing happens to your bitcoin. Your seed follows an open standard (BIP39), so any other hardware wallet can restore it. In a multisig you also still have keys from two other makers.
Your seed isn’t theirs
The 24 words follow BIP39, an open standard that almost every hardware wallet supports. If a maker disappears, you restore that seed on another brand and carry on. This is why the guide asks you to pick 24-word backups instead of formats few other wallets support, such as Trezor’s 20-word default.
If the device still works
Keep using it. A hardware wallet doesn’t need its maker’s servers to sign. You’ll miss firmware updates, so plan to replace it within a year or so.
If the device breaks and the maker is gone
- Buy another brand that supports BIP39 and works with your wallet software.
- Restore the 24 words on the device, plus the passphrase if you use one.
- Register it in your multisig and check the fingerprint against your notes, as in replacing a lost device.
What about the software?
Sparrow is open source. If the project stopped tomorrow, the last version would keep working, and your descriptor imports into other wallets such as Nunchuk. Electrum can rebuild the same wallet from the three public keys. That’s another reason to keep the descriptor safe.
Why three brands help here
If one maker goes bust and its device breaks the same week, you still have two working keys from the other two.