Dominaria can flavorfully support several large sets in a row, but Dominaria has proved that it can be made to mechanically fit into one set quite comfortably.
Eh, what does that even mean? I'd argue that's impossible to quantify objectively, especially in the case of Dominaria. Like, sure, the mechanics that are actually in the set... fit in the set. Just like every other set. You can't really compare the mechanical demands of visiting Ravnica with Dominaria. Let's have a look at what the mechanical demands of returning to Ravnica actually are. I think we can all agree that you basically have to include all ten guilds, each with their own keyword or ability word, with enough cards for each guild to support it. Ideally, you'd also have room for the mono-coloured bread and butter cards that keep it all together, fill certain roles for Standard etc., plus a few oddball cards that give players alternatives to build around, e.g.
Ratcatcher or the Nephilim cycle. And yeah, it's easy to see that you would need either one large set followed by two small sets (the traditional block structure) or at least two large sets.
Now, what about Dominaria? Sure, this time they decided to go with a history theme (and they'll probably keep that in some form going forward), and sure, Sagas and historic matters were cool ways to represent that. But honestly, what we actually saw in
Dominaria as far as representing the plane as a whole is concerned was basically the equivalent of going to Ravnica and only showing two or three guilds. And that's true for both worldbuilding and flavour as well as possible mechanical identities. A plane that is so vast and that can be almost anything - even with an obligatory history theme layered on top of it going forward - can't be adequately captured in one set. The things we
did see in
Dominaria are dwarfed by all the things that we
didn't see. The fact that Dominaria has so many different things to choose from means that, yes, it can be handled with less space than Ravnica could if it needs to be, but it also means that it could effortlessly support an almost arbitrary number of large sets in a row without getting stale and boring. I distinctly remember playing Magic at a time when we spent six consecutive sets in Otaria alone and it was just another day at the office. And that was after almost a decade of "all Dominaria all the time forever".
Maybe Ravnica can support the upcoming block, but I can guarantee you that after that, people will be like, "please, no more Ravnica" for a very long time. What's more, time is an important factor here. It's been five years since
Dragon's Maze came out, but eleven years since we last saw Dominaria (before
Dominaria). And yet Ravnica is getting three times the exposure that Dominaria does now. That's insanely frustrating.
But as I said, I won't be around for the new Ravnica block. When the dust in the wake of
Dominaria has settled and all is said and done, I'll go on an indefinite hiatus from buying sets, and I'll stop following this so-called storyline, just as I said I would. I've been sick and tired of plane hopping for years, I've hated the Gatewatch from the very beginning, I refuse to be cool with the fact that my new cards are curled the day after I opened them (ironically, the most expensive ones from the Masters sets also curl the most), I consider the short story format a failed experiment, I want the continuity to sort itself out and this mess of a story arc with all its ruined characters and retconned plotlines to just go away. I love
Dominaria, and I'm turning my back on Magic in spite of it, not because of it. They've proven they can get it right if they really try, and I'll gladly reward that with my money, but it does nothing to extend my customer loayalty beyond this one set. However, it has given me a glimmer of hope that maybe I have more Dominaria to look forward to, and that it'll turn out fine. And that's something I didn't have before. We'll see how it goes without Kelly and Mark Winters, though...
That said, I still stand by my suggestion of a three pillar model in which Dominaria is treated as its own category in addition to new planes and returning planes. Magic needs something like a homebase again.
Edit: Oh, and I almost forgot: Dominaria only getting one set has also proven to be disastrous for the story it's trying to tell. Mechanics and worldbuilding are important, but they aren't the only important things.