Is 16gb enough? I have two extra slots and memory is inexpensive, but is it a waste of money?
Thoughts?

It could be, depends on the plug-ins used and the size of the projects. Large projects using a lot of sampled instruments, especially if the plug-ins do not stream samples, may benefit from more RAM.

Now that samples are often put on SSD drives, I wonder if most samples are now being streamed?  Is this the trend?

I have 8GB and have never even nearly run out. It's true that large sample collections use more RAM but my use is limited to pianos, rhodes, drums, bass and sometimes something like Session Strings Pro or Hollywood Orchestra Gold strings. If you do orchestral stuff and use a lot of the latter, it would probably help. 

Theoretically the more RAM that's available, the less demand on the page file (virtual mem) and thus less hard drive use. Right?!

Not necessarily. The page file by default is twice the size of physical RAM. More RAM means a larger page file which Windows always uses for some things unless you specifically tell it not too. It's true that too little RAM can mean the page file gets used excessively though.