I'm actually very much in favor of monthly fees.
If you purchase a "lifetime" script, and the scripter decides to leave in 3 weeks, what are you going to do? You have no protection against that. You could ask for a refund, but you'll probably not be eligible anymore. In addition, you can't expect a "lifetime" script to be for your lifetime, because... well, the scripter could get sick, he could get run over by a car, he could have irl issues, and end up being unable to update his script.
On the contrary, with a monthly script system, the scripter needs to keep his script updated to get paid. No updates, people don't renew, no income. Therefore, monthly payments are an incentive for scripters. In addition, scripters have to keep their script updated, fix things when Jagex breaks their script, and they should get compensation for that work. And please don't pull the poor kid argument... If they have enough money to be spending 8 bucks a month for a video game, they have enough money to spend 5 bucks a month to pay the scripter for his/her script...