I agree.
Data, it comes down to data / verifiable proof. Obviously, that depends on the budget available.
Of course there is no shopping cart, although leave it up to groups like Vroom.com, etc. and that might start to change (younger generations seem ok with it -- I don't know if that's smart in the long term, but it's gaining momentum) and if perceptions change, so will dealers (through on-site and on the lot / showroom sales strategies)
Obviously, usability / UX testing can easily be applied to lead conversions as well (which I think you're alluding to). It doesn't have to be measured through an actual commercial sale / e-commerce, but definitely can be for acquisition % based upon leads. It's about taking advantage of your traffic and converting it using a qualified sales team. I'm sure you know, scores can be applied to leads. If quality, intent based leads shoot through and a sales team is unable to convert well, that tells you a lot about your sales team.
This process has already been defined by many 3rd party groups, that's the thing. It's not about defining it internally (that's a cluster fuck of an idea -- dealers looking to cut costs in the wrong spot). It's about using qualified 3rd party groups to do it for you. Honestly, I am seeing it within website platform services, such as Dealer Inspire, etc. They are moving in that direction.
I am quite surprised there aren't more usability groups partnering with platform and / or CRM providers. What I see at Cox, etc.... buy up groups and create a seamless piece of technology for dealer groups.
Sadly, there are still loads of platforms that completely suck and do not take usability seriously. Garbage inventory systems (SRP, VDP), that barely function on all devices and browsers.