Xbox Live has been around for longer and they had no problem naming their new services a different name and getting people to sign up, both first timers and upgrades from Live Gold. Game Pass is a descriptive name for the service and "Ultimate" makes it pretty clear there's nothing higher. I'm heaping praise on that despite me thinking their console naming strategy is asinine.
To be honest I think you're nitpicking. Here are the names:
PlayStation Plus Essential
PlayStation Plus Extra
PlayStation Plus Deluxe
PlayStation Plus Premium
Again, it doesn't take any real thought for someone to look at those names and realize that "PlayStation Plus" is the base name and the different tiers are based on the last word. And as I noted above, it doesn't take much thought for someone to denote their meanings just by looking at the 3 tiers that'll be available in their region.
And all of this is assuming that this hypothetical someone just spends their time looking at the names and that's it. That they don't take a couple of minutes to see what any of these tiers offer and what differentiates them. The people you're describing will be in the extreme minority and are not in any way, shape or form worth ditching an incredibly successful branding just to cater to them
Sony's ditching the PlayStation Now branding to take advantage of the far more popular and widely known Plus branding. It's the right move and makes FAR FAR FAR more sense that killing the Plus name and then trying to re-educate all ~50 million people that the service they've been paying for is now called *insert name here*.
As for Xbox and what they chose to do, who cares? They chose a path that they felt was best for them and so has Sony. Sony felt that keeping the widely known and recognized Plus branding was best for them and it makes perfect sense. Hell, with other companies like Disney adopting the "Plus" title it just makes it even more recognizable to the general populace and helps give them an idea right off the bat what it is.