Following that line of reasoning down the slimy slippery slope, developing better encryption systems could be construed as obstruction of justice, no?

No.

This is exactly the point he was trying to make...

No, that's not the point I was trying to make. The people in Washington D.C. that sit in fancy leather chairs have repeatedly demonstrated their lack of critical thinking skills, so I use terribly bad sarcasm to illustrate what their puny brains will think of next.

How can you be so sure?

Interesting point. AFAIK they requested the key to decrypt previous communications. From security point of view, his move makes perfect sense. If FBI wanted to decrypt future communications, they would probably have specified that in their request. Then he would indeed break court order and could be hold responsible.

On the other hand, it would be hard to prove any actual obstruction, since the service was shut down and all users notified about this whole situation.

Who would make future secure communications with Lavabit at this point? Certainly not Edward Snowden.