As mentioned earlier the Joe Casey book ‘Sex’ is supposedly following that model (but the first OGN hasn’t come out yet). Love and Rockets has too, League of Gentlemen to a degree as well went from mini series to OGN.
It’s not financially unviable really, it’s how prose books work and have for years, which is why the Franco/Belgian market supports it. It’s more the element of the unknown and extra risk moving to that model.
Right now by all accounts with an Image book if you sell 5-6k you kind of break even. It gives an immediate indication of interest and trade sales are really all gravy as you’ve covered the creator costs. It’s hard to lose significant money.
With an OGN model you could work for a year and get nothing if nobody is interested.
Prose publishers and the European album guys are attuned to that risk and it’s part of their model. They know they have a guaranteed level of income from timeless classics, be it Harry Potter or Tintin and then hedge bets on the new stuff. Most new novels fail and sell nothing.
I can see it’s hard to make the jump from one to the other. For the likes of Top Shelf they know Alan Moore is going to sell a lot anyway, they did a first run of 75k copies on the LoeG OGNs and sold out first day. A new book from Jody LeHeup & Nathan Fox is another matter.