When I worked elsewhere, and had more of a choice, I always chose to walk in winter. Since leaving Spain in 2006, I've worked exclusively in roasting hot countries, and I always loved the change. Now, I'm working in a country where my only possibilities for getting a sizeable chunk of time off fall in winter - early spring, so I can carry on with what I prefer.
I don't doubt that winter pilgrim numbers are up a bit, since there are people with time flexibility seeking to avoid the masses, but I don't think it will ever reach significant numbers. I know there are a few people on this forum who did their first camino earlier than I did, but in 1998, in Nov-Dec, I met 14 pilgrims in total, when I was either going to, or returning from SdC. There may be more now - I'm sure there are - but I don't think there will ever be enough to put me off. I've only ever walked once in 'high season', and I was doing the Norte before it became super-popular. Everything went well until the very last couple of stages, and the American pilgrim I'd met up with along the way, with whom I met up every night, told me before the final stage, where we had to join the CF - 'you are going to have the shock of your life'. She was right. That final day on the CF was horrific, and not to be repeated. Viva el invierno, say I, and may it not become toooo popular!