To answer your questions, it depends what they are asking. Typically when we talk to senior stakeholders we are using scenarios based on current performance. If you are trying to justify additional spend, you might want to talk about what the business would look like if the visitor to lead conversion rate increased from X to Y - looking at how many additional customers that would create based on current closed rates. If defending current spend, you probably want to look at recent traffic, lead and customer trends and talk about the effect of those going into decline.
I would really encourage you to do your own internal benchmarking. I know different sites in the same industry that have widely different conversion rates. Why should you look at the industry standard when you should be focusing on improving your results. If you have a site that outperforms the competition that is no reason to not look to increase that performance.
Our agency works with clients to improve the overall impact of their site with a continuous growth driven design process. You can check out the method here and implement it on your own site: https://www.weidert.com/growth-driven-design-websites
It looks like the outtake from all the reading including the info you have provided (much appreciated), points to that conversion rate benchmarking is a dirty phrase. There are too many varibales to rate the throw away 5%-10% conversion is where you want to be.
Perhaps then my next question is more around how you educate and provide context to C-Levels on conversion rate performance to combat questions around Marketing Spend Vs. Conversion.
To answer your questions, it depends what they are asking. Typically when we talk to senior stakeholders we are using scenarios based on current performance. If you are trying to justify additional spend, you might want to talk about what the business would look like if the visitor to lead conversion rate increased from X to Y - looking at how many additional customers that would create based on current closed rates. If defending current spend, you probably want to look at recent traffic, lead and customer trends and talk about the effect of those going into decline.