Many issues are pie in the sky enhancement / feature requests that may or may not be implemented some day. There is no intention to drive the number of open issues to 0 just for the sake of it.
As long as Element and Matrix continues to grow, I expect the number of open issues will grow indefinitely, and that is a sign of success.
Just looking at their GitHub, they have an ridiculous amount of 7000+ open issues (5k on Web, 1k on each Android and iOS). Assuming they'd use a generous 1/4th of the money to fix outstanding bugs, assuming no duplicate bugs, and assuming a 100$/hour salary for the developers, they can spent on average about 10 hours to solve each bug (30000000$/4/7000/100$).