You don't: You get money for having saved money rather than spent. Consider that you would have to lose out on 20 upgrades at 5% interest to get just 1 per day. Its a long term investment and it would take 20 days to start really benefiting you.
This isn't very much and would be very slow (it would take a couple of weeks for a new player to save 30k and then he would have to wait 1-2 months to fully upgrade his deck by doing nothing, comparatively you can earn 1400+ per hour farming and 4000+ per hour FG farming.
I think it would work well, the exact numbers (% interest etc) could be determined by a detailed discussion
Edit: fixed some typos.