I do think it's still really too early to be taking the new email tool seriously as an option for real-world production. There's still a bunch of features that are lacking in the tool that limit the functionality compared to the existing email editor (v2.0). Even once the "bugs" are worked out and all the road-mapped features are added, I'm still skeptical that it'll be something you can trust. The real issue with WYSIWYG editors and automatic code migration (for templates, etc) is that someone somewhere has to make decisions about how to interpret the HTML and force that into some sort of standard format programmatically -- that's difficult for even seasoned email developers on a case-by-case basis let alone as a single program/format/logic-path and I haven't (in my 10 years of Marketo Email development) ever run into a migration project where there weren't questions or concerns around how to handle taking "some other code" and translating that into Marketo's Email syntax. At the end of the day, it's just not a one-size-fits-all world.
Moreover, this kind of tool needs constant baby sitting -- as things change in the email code landscape (which they often do) things that worked yesterday no longer work today or things that were once best practice are now old-hat. From what I've seen so far from a technical perspective of the code base, I don't trust the tool or the framework to produce consistent enough results to compete with what we're currently doing in email 2.0 -- that's to say it's technically a step backwards. Since the code base is locked away and not something that is open to developers like the email 2.0 model is, you're kind of stuck with what they give you and are forced to align your business requirements around the tool instead of the tool around your business requirements.
When Email 2.0 was announced, it took a few years for it to actually gain traction and even longer before the "old version" was sunset. To this day, you can still make FreeForm Landing Pages in Marketo and that's an ancient feature that hasn't ever gotten sunset, so I wouldn't buy the hype about needing to be ready for the current email system to go away -- think about how many folks out there are using it and how disruptive it would be to hit the kill switch on something like that... it sounds like an easy recipe to lose a lot of business if you ask me.
For my own stuff as well as in a consultative manner, I'm thinking late 2026 or 2027 might be a more realistic timeframe to take something like this seriously and even then I'll need to look behind the scenes at the underlying code base before I'm ready to trust a WYSIWYG email tool b/c email is notoriously fickle and not something that's easy or consistent enough to nail down in an automated tool like this without having a lot of "yeah but it doesn't work here and here and here" disclaimers.
Remember - you can get 2 of 3: Fast, Cheap and Good --- this is promising Fast and Cheap email design, so I guess we'll have to wait and see how that all works itself out.