Or impossible, even.
Thats why I just do it. If somebody wants proof that my years of experience applied in a particular way will provide a quantifiable benefit that is impossible I will just avoid doing that thing.
Or more likely, move somewhere which can appreciate those non quantifiable benefits.
How can you find such a place? This definitely resonates with me.
There's so much value in doing things right, but it's impossible to prove. I can do things that make my team go faster, but there's no way to say that having a X% better tests resulted in us being able to release Y% more features.
I'm not going to pretend it always worked for me, but it certainly got easier once I could demonstrate a wealth of experience.
There's a lot of luck, too.
In terms of refactoring, I have an iron rule that I never ask for permission. If I think it needs doing I do it. I don't justify it to product or ask for their opinion. Technical issues and priorities are on a need to know basis and they don't need to know. All the work gets baked into feature work and bugfixes.
Proving the value is the hard problem. It's almost impossible to prove the value of small changes today. Product wants things to do X and they decided the value tradeoff was good, not the engineers who are tasked with making it so.