[Tallest]

Whenever we drive behind a car that has too much undercarriage showing below bumper, my wife always comments how that's bad design. I see it now too. So I totally get where youre coming from.

As for diffusor being crappy glossy plastic - blame it on all shadowline fans who 20 years ago were after those glossy trims that only higher end cars got.
BMW realised that they can charge extra for what is essentially polished plastic today. The original shadowline was made differently, it had a lacquered feel to it. And it tended to suffer from UV discolouring over time, and looked pretty bad. After market came with 3M products, and people could wrap it in tape and get a similar look for way less. BMW figured this out... that by painting 3% of the car surface less and using plastic parts now, they save a ton of money in the long run. Sucks for us. I would only go chrome trim today if I can. Old folks had it right all along.

So that geezer monologue aside, my take - paint/wrap/replace diffusor w something more durable. As for undercarriage no one unless they are in a low sports car can see it, so save your OCD pain and focus on something else.