Yeah that what I thought when I got my first set of plates and it's going to cost me a shit load more now to get thinner plates. Should have just got the proper stuff from the start.
If your serious about lifting buy quality once then your not forever replacing sub standard shit.
6.5cm for 15kg plate won't take you long to fill up a bar.
If I had to replace all my plates I would get a shit load of 20s and then 2x10kg, 5kg, 2.5kg, 1.25kg. Makes it simple, 25s and 15s are a pain in the arse.