If BR goes to 50 HY, I cannot see Big's Spiral being built. This is an enormous site which could support two 1.5m sf towers.
If Tishman finds a 500k sf tenant, it could start one tower with 1m sf of office and 500k sf of hotel, apartments, and retail. It could start tower 2 under a similar scenario.
I don't think that Tishman could let this site sit empty for ten years with the hope of finding a 1m sf tenant. It spent over $600m to acquire the site and air rights, and the revenue from the garage could never service its debt.
This site is physically enormous. The garage alone is huge. Two huge towers with green space in the middle would be ideal. That's precisely what Brookfield is doing at MW.