I would say Airtel & Vodafone are "Fishing in troubled waters" since, now they are just trying to improve their ARPU figures by shredding low ARPU users or forcing recharge upon them. I totally agree with you regarding the burden of maintaining such huge network but it is not going to help them by taking this step as by the way of loosing customers they cannot reduce their maintenance expenses but instead more calls will terminate to other networks.
Though it is going to fetch them some quick revenue but is surely going to affect their future business, since many lifetime validity users were suppose to recharge a minimum of ₹200 every 6 months to continue their services and now forced recharging for them will only yield ₹55 extra ( since ₹35X13 = ₹455 i.e. ₹55 extra for 13X28 days= 364 days). No customer will ever want to live with the fear of losing outgoing after 28 days without any grace period if he forgets to recharge on the last day.
IMHO it is a short term revenue collection policy and TRAI is equally involved since this violates the TRAI's 90 days disconnection policy.
I am just concerned about the majority of lower income people using mobile - as they are going to be affected badly by this move.