They would NEVER tear it down. Its value is in its close proximity to so many other networks. I would guess single digit milleseconds to Wall Street (certain trading firms care), along with interconnections to many mobile operators networks, VOIP providers, etc.
They will want the current tenants to stay, so that Google will have top-shelf speed to all their customers' networks at a low cost.
They also get to collect the rent from these firms instead of paying rent for their current and future space to the old landlord, which will make the financial part better that "Google pays 1.9 billion for dumpy building!" sounds.
They will want the current tenants to stay, so that Google will have top-shelf speed to all their customers' networks at a low cost.