That's already happened. On the SM demos, of course everyone doing the demo always asks for a donation. At the door (at least in my experience), that rarely happens. The only time I ever asked is if I got stuck at the door with the CO. Usually, the only time I've received donations is when the householder would ask how much the rags are, I'd say, we leave them without cost but do accept donations, and they would generally give a buck for them. Out of say 1000 mags I've left with folks over the last 10 years, I'll bet I maybe got $20 total.
If the WTS was making any more on the mags, it would be because they pressure JWs to donate for the literature when they pick it up and then drop anything they receive in service in the box as well. But, that works on the assumption that JWs will donate the same amount (plus inflation) as they did when they "paid" for the literature. Having dealt with congregation books (mainly auditing) up until earlier this year, I don't think the funds sent to the WTS have increased since they went to strictly donations and when you take inflation into account, they've likely dropped.