I agree it's beneficial that it's already silicon-proven by time most customers get it. The biggest selling point, though, was what ARM themselves said when open ISA topic first came up: the massive ecosystem, esp software and dev tools, makes the licensing worth the money. Im not sure how true that is. Probably varies a lot depending on who is using it but might be true for many. RISC-V needs to get its ecosystem super-strong to challenge ARM in general sense on top of pre-proven IP customers can trust. Ecosystem is the bigger issue, though.
I like to point out the difference between having the best engineers money can buy versus having the best engineers no money can buy.
People who work in open source software are usually the second kind. I expect open hardware to attract people with the same motivation to push boundaries regardless of any other factor.
By saving that $1 and by not buying IP from other, you will need to invest it in time, in engineers, in know-how, in support, in verification and so on, all by yourself.
It might not necessarily be a good cost-benefit situation.