Well, it seems miracles do happen. According to the 7.9 testing release notes, Mikrotik will finally support ED25519 host keys. But, is this even important? I would argue yes.
First of all, ED25519 keys are MUCH shorter and significantly faster while providing higher security margin than 2048-bit RSA keys. If you want to use the same key to centrally manage your network and you have some underpowered clients, you will definitely feel RSA slowness when establishing connection - especially when dealing with high-ping situations. And shorter keys are not anything to frown upon either as they get much easier to copy/paste than wall of text RSA provides.
Secondly, security of ED25519 seems quite robust and sits somewhere between 2048-bit and 4096-bit RSA key. Unless there is a major breakthrough in cracking ED25519, this is good enough for foreseeable future. When/if quantum computers become a reality, both RSA and ED25519 are fcked so you're in a losing battle. However, ED25519 keys seem to have a quantum-resistant NTRU-X25519 key exchange in OpenSSH while there is nothing similar for RSA.
Albeit I'm not cryptographer, I do listen to a lot of smart ones and most of them assume any quantum scaling breakthrough necessary to break ED25519 keys will buy a few years at most for RSA algorithm. In short, while both RSA and ED25519 may be doomed at undefined time in the future, it seems unnecessary to avoid faster algorithm ED25519 is today.
Lastly, for me this will mean I can use a single management key once more as, at this time, I'm using ED25519 for most of my needs with RSA being exclusively kept for the purpose of managing Mikrotik. Finally, I'll be able to use one key to rule them all.
Good luck with upgrade!
[2023-05-04: Unfortunately, ED25519 support is partial at best. If you try to assign key to a user, you'll get unable to load key file (wrong format or bad passphrase]
[2023-08-18: Well, while ED25519 support has been with us since 7.9, one couldn't import any ED25519 keys. If 7.12 beta 1 release notes are to be believed ("ssh - added support for user ed25519 public keys"), we should finally have it done fully and properly. Let's see...]
[2023-11-15: At last, ED25519 is supported by Mikrotik as of RouterOS 7.12]