I would not say we are anywhere close to perfect in compilation. Even just looking at inlining for size, there are multiple recent studies showing ~10+% improv

I would not say we are anywhere close to perfect in compilation. Even just looking at inlining for size, there are multiple recent studies showing ~10+% improvement (https://dl.acm.org/doi/abs/10.1145/3503222.3507744 https://dl.acm.org/doi/abs/10.1145/3503222.3507744, https://arxiv.org/abs/2101.04808 https://arxiv.org/abs/2101.04808). There is a massive amount of headroom, and even tiny bits still matter as ~0.5% gains on code size, or especially performance, can be huge.