It'll definitely be an interesting adventure for all involved, but I don't think it's a surefire failure.
For one, the Ducati is also anything but stable, planted, or consistent, and he just handily obliterated the whole field, including his far-more-experienced-and-successful-on-the-same-bike teammate. Crutchlow has been taught for much of his career to ride like Lorenzo, and he's also been decently successful on the Honda.
It's possible the finesse Lorenzo has, which may very well be second to none, will help him tame the Honda. After all, Pedrosa is no more predisposed to do well on it, and overall he certainly has.
I also think that a confident Lorenzo is a force to be reckoned with, regardless of the bike he's riding. When he's down, he's nowhere to be found, but when he's up, watch out for the holeshot. Never underestimate a card-carrying alien.
And yes, part of my optimism is hope. I don't necessarily like him, but i love great competition in the sport, and any challenger to Marquez is a welcome thing at this point.
After all, who are other title contenders for 2019? Rossi and Viñales, maybe Dovizioso again, and that's probably it. Zarco is great, but the KTM will probably need a few more years to really be up there. Suzuki has talent joining, but neither the riders nor the bike are of that caliber (yet). Petrucci is a nice guy, but I don't see him contending for the title. Same for Pol Espargaro. Aprilia is only nominally closer to running at the front than I am. Satellites are too second-fiddle generally, and independent teams are filler. (If Pedrosa somehow lands on a factory-supported satellite Yamaha, I expect him to do well, but probably not that well.)
With Lorenzo on a competitive bike that makes for five realistic title contenders on three different bikes. Importantly, he'll have access to the bike that's effectively dominated for years now.