You can't measure the impact of not creating a steaming pile of complexity.
Really you can. You look at the engineers who create steaming piles, and you look at the ones who don't. Over a year or two, the difference is easy to spot. For people who care to spot it.
If there's no competent front-line technical management who can successfully make this simple comparison, then, sure, in that case the team may be fucked.
Measure no, but only engineers care about that (and I'm not even saying that they're right, engineers care a whole lot too much about hard data). You can show alternative solutions, estimate, make assumptions, even make up numbers and boom, you have "data" to show you improved things. You don't even have to lie: you can be very open that these are assumptions and made-up numbers, that it's just a story, what's important is that people come out with confidence that thanks to you, things are better by a bit/a lot/enormously.
The impact is that you get to go solve another problem. This absolutely does show up in a good performance review.
You can. GitHub is about to hit zero nines of uptime[0]. But feedback like that is far too late to be useful. Maybe (principal or senior) engineers should be the ones to judge, and be trusted by management that their foresight is worth pushing the deadline?
Won't that show up in roi numbers?
One of my the papers I share around a lot is "Nobody ever gets credit for fixing problems that never happened (2002)"[1]. I like it because it's not purely about software so the examples resonate better with some exec level people in other teams I work with.
[1]https://ieeexplore.ieee.org/document/1167285