I'm not convinced it was doomed to fail - isn't this the nature of every commercial landlord/tenant relationship. One side wants to pay as little as possible, the other side wants to charge as much as possible. Neither side, typically, wants to break the other though.
As an aside, this 'sustainable' thing is an odd argument to me. If you can afford to pay millions in salaries, you can't really claim that the rent makes your business unsustainable.
ACL though, did have an unavoidable fixed cost, the mortgage to YB. Even SISU could see that a large part of the driver for the high rent was the mortgage, hence the roadmap which talked about buying it out. One of the many ironies in all this is that refinancing that mortgage with the council allowed (or so it was claimed) ACL to lower the rent.
Anyway, in truth this has been done before too. The more I look at this the more I think we just need to move on past it all, and hope that after the dust settles the serious talks start.