I think the 10k limit is there to prevent large sums of money being parked at bunq just for the interest. Where old school banks can profit from the interest on loans, bunq doesn't. Therefore it would cost too much to give unlimited interest, when there is no opposite income.
Bunq stores most of our money on their account at the ECB, where their account carries interest. As the interest is very low on those accounts (negative 0,4%), it would cost bunq a lot of money to park huge sums there. Unlimited interest would attract just that: large sums of money.
The cap prevents too much money being attracted, whilst the interest may attract small sums of money, e.g. attract paying customers.
So that may be a reason why there is a cap on interest. When the interest on bunqs ECB-account increases, or the profit on investments rises, it can be a different scenario. Bunq might make profit on our money and may or may not share that profit with us.
In the end bunq is a commercial company, which aims on making profit. They have the subscription fees, the interest on their ECB-account and the investments.
