Do you think all those ASP.Net sites are more likely to be delivered on the web to a diverse selection of evolving, modern user agents, or on an internal network to only IE7 users until 2019?

One opportunity has a choice in development tools. The other doesn't, really. The former market for developer mindshare can and does change; the latter is Microsoft's entrenched base which is never really in jeopardy (nor very relevant to the growth of the tech field at large).