Since there seems to be a reluctance to charge for card topups, how about providing a positive incentive for using Monzo without card topups. Perhaps a lottery of some sort?
All Monzo users could get entered into a monthly prize draw (perhaps you could re-purpose the now defunct golden tickets for this) but lose their entry in any month in which they do a card topup. The prize could be relatively small from your perspective but large enough to incentive individual users to change their behaviour - perhaps £500 to £1000 per month prize.
Whilst the above has a very simple set of rules, a problem is it would potentially reward inactive accounts. So perhaps you also put some barriers on gaining the ticket in the first place. Perhaps users need to have x number of transactions in a given month to gain a ticket. Perhaps users who also have their salary paid directly into Monzo get an extra ticket.