Wow.

I can't believe a bike "mechanic" would make that statement. I commute on cantilevers all the time, and I've ridden a ton of hard mountain biking miles on them too.

The key to your problem is your "ancient pads." They don't look like they were very good pads to begin with, and over the years the rubber has probably become very hard. As noted several times above, a simple pad switch should have you stopping fine.