Their statement is that "As a special offer to Visa Electron card holders, Ryanair, for a limited period only, will not apply a Debit Card charge"
I suspect it will remain free as long as bookings using Electron cards remain below a certain percentage of overall bookings. If more and more people move towards Electron cards (quite difficult given it's limited availability worldwide) then they will have to apply a charge to consolidate revenue. The downside to Ryanair being having to include this charge in their advertised fares, the whole reason for having it free in the first place.