One of the major drivers behind the decision to switch threading models was
this: to make the SA implementation work and be scaleable in an SMP setting
would have required significant reworking of the kernel components. That
would have meant, essentially, a rewrite of the core in kern_sa.c bringing
its own new bugs and headaches.

Sometimes reliability and correctness have to take precedence over other
concerns. It is a lot easier to implement the missing pieces for 1:1, and
have a far better prospect of having a reliable, well understood and MP
capable system in short order.

One or two of the advantages that SA provides, like reduced use of kernel
stacks can be gotten in other ways. I'll be looking at improving the 1:1
implementation as time goes on and I'm sure others will be too.

Cheers,
Andrew