Victim of their own success?
Personally I think the Hypervisor is the new OS analogy and likening VMware's growth to that of DOS -> Windows in the 80's and 90's is a little flawed.
The "market share creates more market share" feedback loop worked for Windows because developers had to make a choice about which platform they were going to develop for. Once MS had a certain portion of the market, if you were a software house looking for the best returns on your product development costs, it made sense to focus on OS with the majority market share rather than burden yourself with the expense of developing for several different platforms.
On the other hand, the hypervisors greatest trick is convincing the underlying workload that it's not there (not unlike the Devil I guess ;) ). By function of design therefore, any Hypervisor is almost invisible to the virtualised OS, applications that run on said OS and the end user. This invisibility makes the hypervisor relatively easy to replace with one that is better/cheaper/faster/hardware embedded. Management and end users don't care about the virtualisation platform their applications are hosted on. All they care about is that they can collect their email, run their CRM system and browse their intranet. If today these workloads are hosted on a VMware hypervisor, fine. If tomorrow it's hyper-v or some type of hardware integrated virtualisation, so long as everything works, who cares.
VMWare almost single-handedly created the x86 virtualisation industry, and took it from a niche market to the massive industry that it is today.
The only problem for VMware is that in doing so, they have created an awareness of the benefits of virtualisation and a demand for products that can satisfy that demand but that product does not specifically have to be ESX / vSphere. Thanks to the invisible nature of the hypervisor, anyone can have a crack at building a competing virtualisation stack, and if its good enough people will use it. This is not like having to migrate from Windows to OS X. You can just pickup your workloads and drop them on to a new stack. So long as it's reliable, Exchange will keep Exchanging, and users will keep receiving their email.
VMware have already done the hard work of making the market receptive to the concept of virtualisation, now anyone who's competent can build an offering and benefit from the wide industry acceptence x86 virtualisation now enjoys. Thus, VMware risks becoming a victim of its own success.
If VMware wants to stay relevant into the future, they need to do what every good software company does, and find tactical ways of adding proprietary lock-in to their product. And that's a challenge that I think they remain to successfully address.