MSP430 Series 5 Guaranteed Stock
Any answer that we could give you would be pure speculation.  Speculation based on experience, but speculation none the less.  So, here's what you do, in no particular order:


Talk with TI, the rep, and the disty and ask them which ones are more likely to be made for a long time.  They know this much better than probably everyone here on Stackexchange.
Pick a package that has a long shelf life.  I'm talking about the time from when you receive the chips from the disty until you solder them onto a PCB.  Some chips/packages will get old and can't be used after some period of time. Sometimes this is due to the storage humidity, and sometimes due to other reasons (corrosion on the pins?).  Research this and choose a package that will last for several years or more on the shelf.
At least once every quarter, check with TI/Rep/Disty on any end-of-life (EOL) notices.  Absolutely DO NOT rely on TI/Rep/Disty to inform you when it happens.  You need to check yourself.  They will forget to tell you until it is too late.
Plan for the day when you have to buy lots of parts and keep them in stock for years.  This might be when you get that EOL notice, or it might be next week.  This plan should include a place to store it (temp &amp; humidity controlled cabinet?), as well as having cash on hand to pay for the parts.


After going through the numbers you might decided that it's simply too expensive.  It's quite possible that the cost of buying years worth of parts and storing them is higher than redesigning the PCB with a new part.

I should also say that even if T.I. says they will be making the parts for 20+ years you still need to plan ahead.  Things happen despite everyone's best intentions.  In the past, earthquakes have destroyed factories and instead of rebuilding the factory companies have stopped making the chips from that factory.  Same with fires.  Or tsunamis.  While unlikely, it is possible that TI gets bought by another company that decides to EOL the entire line.  You just never know-- so plan ahead.