Microsoft definately does have some plans for Beyond R2. There have not been a lot of public remarks however. Based on some of the conferences that I have attended, the next version of BizTalk will be based upon Windows Workflow Foundation. One thing that I have noticed though is that Microsoft will provide you with the tools to provide integration inside or outside of BizTalk. Whether that is WCF, WF and with the BizTalk Line of BUsiness Adapters coming out next year that do not require people to use BIzTalk even though they are called the BIzTalk Adapter pack could provide for some interesting solutions. The value add for BizTalk is the built in plumbing(guarenteed delivery, message persisitance, message retry, security and scalablility to name a few. BizTalk has a good story around this.
In terms of market penetration, Microsoft has over 7000 BizTalk customers(I would imagine this is all versions combined) and 90% of the top Global 100 use it. So as you can see Microsoft has no incentive to let it die. In terms of what type of industries use it, I can tell you that BizTalk has a presence in these industries Oil and Gas, Financial, Health, Government, Manufacturing, Utilities and Entertainment.