>> There's little/no penalty to jobhopping frequently in this field
First point, there absolutely is. I review resumes, if someone has a history of moving on after a year, I assume they are going to move on after a year again, and since it typically takes that long for someone to be truly effective in the environments I have worked in, that's a hard no. Nothing personal, but no. With a job market as good as today's that might not be a huge penalty, but it's there.
Which leads us to the second point, people who are doing this are playing a game of musical chairs. If you have been in the software dev world for a few decades, you have been through some rough job markets and you know another one is coming, you don't know exactly when. You have seen people much smarter and more capable than you walked out the door, and not be able to get a job for years. You have seen half the company you work in get walked out the door suddenly one morning. And when that job market hits, the selection standards of employers suddenly gets much higher.
Just a friendly warning in case this is your first rodeo.