It won't adapt to new windows or directx versions
Well, the engine of the old core is made for DX8. There are no big changes in DX8 in the future. The new core is made to work with DX9. Which also will not have this much changings anymore in the future. SO while this is true it doesn't really matter. And an exported mesh doesn't care about the DX engine of tS anyways ...
it won't be able to make use of new hardware
Possible. Works currently fine at XP and Vista though, plus all graphics cards that supports DX9. And there is a chance that it will also work with Win 7 then.
it won't have any new features the competition will come up with
As told, Plugin developers are still at work.
In a few years it will be useless and you will have an outdated skill set
As told, a vertice is a vertice. An endgeloop will stay an edgeloop. And it will be a edgeloop in 100 years too. I make my graphics now, not in a few years. In a few years i may have a look for another tool then
How can plugin authors fix bugs in the closed source core?
trueSpace has a very open modular architecture. They can simply write scripted replacements. Not possible in all cases, but in lots. One of the plugdevs has already fixed some quirks that were introduced by the rushed release. One example: broken weightpaint is back working. I expect to see more than one fix be done that way by plugin developers.
What a waste of time to work on plugins which will result in a patched up mess. :p
You would be right when it would end in a patched up mess. But i don't see that happen. What i see is that Plugdevs finishes and expand what Caligari has started.
A good example is the VRay renderer story. Caligari has removed it, the Plugdevs have plugged in Yafaray renderer then as a substitute
Milkshape doesn't progress either (actually I would call it dead too) but at least its author is still around for bug fixes as far as I know. If you are into simplicistic and limited 3D modeling and animation software then there are better alternatives like fragMOTION.
Neither Milkshape nor Fragmotion is an alternative for me. The tS bones are useful and stable enough for my needs now that IK 2 is implemented. And i wouldn't call them simplicistic or limited. Still a bit buggy though, yes. But i can live with that.
Let's talk about the pros. There is, at least at the moment, no other 3D package with a this good realtime engine implemented. You always need to load your graphcis into your game to have a look how it turns out. tS Workspace works with DX shaders. You can directly have a look how it plays together.