I've spent a healthy chunk of time analyzing this and have taken a position.
By my best and most conservative estimates, Sears would have to both declare bankruptcy and reject all of the leases in the portfolio within the next 6 to 8 quarters in order to put Seritage in a bad situation. And even then, as long as SRG could access external financing at a reasonable rate of funding, they should be able to survive without too much trouble. Beyond 8 quarters, the company should be able to generate positive cash flow even if Sears suddenly disappears off the face of the Earth - the one large assumption here is that they're able to continue to recapture and redevelop properties at the same rate that they have been for the last few quarters. If that suddenly drastically slows then it's a different story. So that's the downside scenario.
The upside is massive, and I think will happen more quickly than most people expect. A complete turnover of the portfolio (meaning Sears completely gone and new tenants brought in) gets me a price target of ~$224 using an FFO model and $204 using an NAV model. By my estimates this should take somewhere around 15 years, giving us an annual return of ~11%. This doesn't take into account any rental inflation, any growth in the portfolio, or really any other sort of excess return that could be generated by the management team. This is a pure, steady-state portfolio turnover. So I think there's probably upside even to my estimates.
For anyone who is concerned about the near term Sears bankruptcy risk, it is somewhat cost effective to hedge a position in SRG by buying long-dated SHLD puts. The Jan '18 expiration covers most of the risk, as by my estimates the risk should be greatly diminished past that point and an investment should no longer need a SHLD hedge after that date.