I have wondered this as well. If I kept to the SW plan forever, way past target, without increasing my HEs to maintain, would I continue to lose weight?
I think not. I think the body would keep losing over time, but as you get lighter, your body requires less calories to maintain itself day-to-day, and so eventually I think the weight loss would tail off and eventually stop. That is, of course, unless you start eating smaller portions, less snacks or whatever, in which case you'd perhaps lose again. The weight it would stop at would be hugely different for everyone!
You might think it's easy to eat the SW way...and I totally agree. It is! However, such is life that things will always crop up that hinder our ability to stay on the plan. Holidays, weddings, happy times and sad times. I'd say there isn't one of us here who hasn't had a little blip at some point, be it for a day or two, or couple of months. We all go off plan occasionally...but the successful slimmers go off plan differently. They still pig out, but often in a more measured, sensible way (compared to how they used to pig out pre-SW!). And the really successful slimmers get over their blip by getting right back on with the original plan...sometimes even looking forward to getting back to eating the 'SW way'! That's certainly happened to me after a fortnight's holiday. I couldn't wait to start eating healthily again and start losing again.