This take makes a very clear argument, but I think it's important to question that, as a company grows, it is generally "willing and able to deal with building owners directly" - a similar argument could have been made for the early days of AWS, when it seemed like a great way for startups to avoid managing low-traffic, scalable infrastructure, but redundant for companies that had the resources to manage their own servers. AWS has shown how successful a 'platform middleman' can be, even for very large companies.
WeWork is betting on taking a similar track - sure, companies _could_ decide they want to build out expertise in leasing and managing real estate space, but it can actually help them focus on their core business more by leaving it to another company that provides the 'platform'. The growth of WeWork's enterprise offerings and general awareness of how to pitch to mid- and large-size tenants indicates that growth projections are not pinned to a startup-driven economy.