I did not respond to any question of your on 5/13/08 (I was not affiliated with JA back then).
IF XYZ entered into an agreement with ABC whereby XYZ agreed to provide ABC with a subtenancy in the landlords building then arguably XYZ shouldn't enter into a lease with landlord that would preclude XYZ from fulfilling its obligations to ABC; I say arguably because even if XYZ enters into an agreement with landlord for space with a clause precluding a sublease to ABC, at that moment in time XYZ has not breached the agreement with ABC until such time as ABC demands performance under the agreement between ABC and XYZ AND
XYZ denies ABC the space (XYZ may choose to breach its agreement with landlord and fulfill its obligations to XYZ) alternatively XYZ may enter into yet another lease with landlord that will provide XYZ with the opportunity to lease space to ABC. Keep in mind that if the agreement between ABC and XYZ does not indicate that the space must be provided in the building in question then XYZ could substitute space at another location to satisfy its obligations to ABC.
As I see it XYZ may have an obligation to ABC to provide space to conduct his business and do with the space as ABC wishes pursuant to the sublease between XYZ as sublandlord and ABC as sub-tenant.
To the extent that you can prove damages for expected earning then you might have a case (if this were to be a new business by ABC then this might be difficult since the probability of failure is high for new businesses and the resulting income is highly speculative).
As for the landlord- the landlord has no obligation to provide ABC with an opportunity to earn an income on his property - perhaps, if there is a cause of action
for intentional interference with a valid business contract (i.e. if landlord does enter into the lease with xyz and does not preclude subleasing to anyone except ABC), then you could attempt to make that case but landlord would be well served to avoid putting a clause in his lease stating "No subleasing to ABC" and instead use "no subleasing".