Recently Merkle Trees have indeed made significant progress. XMSS is already standardized, has an RFC, and my Linux distro is already shipping OpenSSH with XMSS authentication enabled. But I don't think it's something practical that you want people to start using immediately.
First, the threats to digital signature created by a quantum computer is not as serious as public-key encryption. If the NSA has intercepted my ECDH handshake, they can recover my session key in the future. But if, by 2045, the NSA has managed to forge my signing key, they cannot use it to log into my server, or create false OpenPGP-signed statements. Either the server has been taken offline for a long time, or at least key has already been retired. Unless it's a digital contract of paramount importance (Excluding blockchains, do we really have digital contracts with a validity of more than 20 years?), I don't think it needs post-quantum capabilities today.
Second, XMSS and many other Merkle Trees-based system has a serious limitation - statefulness. If the private key is not updated correct after each signature, and the state is ever reused, the private key is exposed for everyone to forge. It means, starting using XMSS today creates more hazards than not using it.
I do see some reasons to standardize XMSS: (1) Unlike public-key encryption schemes, the security properties of Merkle Trees-based signature schemes are well-known, with strong guarantees. (2) Standardizing it early allows people to be familiar with the standardization process, eases the future process of more PQC standardization, (3) Allow people to implement and deploy it in an experimental setup.
But I really wished to see the standardization of a stateless hash-based signature, like djb's SPHINCS-256.
I'm curious about your takes on this topic.