Note that even if we keep the “in perpetuity” phrasing, it’s still a percentage of the block rewards. So it’s still subject to the halving of block rewards, and thus diminishes over time, eventually to zero. Just very slowly. So it’s a matter of pace.
As proposed by @ChileBob in Continued from block rewards with a halving schedule, it’s also possible to decouple the two and have the Dev Fund shrink faster than the block reward.