But unfortunately most devices don't (and can't) run Android 5.0, so they're stuck with the old, cumbersome Wallet (or, worse, tried it, gave up on it because it was a terrible experience, will never try it again).
I could've sworn it worked the same when I was running 4.whatever on my old Nexus 5 prior to the 5.0 update. From an end user perspective I haven't noticed much of a change in at least a couple of years (aside from some vendors disabling support in favor of their own tap to pay).
The difference I believe came with the ability for the app to automatically be run in the background and for control over which one was used for Tap and Pay, which given today's announcement from Google makes sense.
Maybe this was fixed in Android 4.4, but I would swear the experience changed for me sometime right at or after 4.4.
Notably, I didn't have to make sure the app was open and wallet unlocked first, whereas with Android 5.0 it "just works".
To be fair, the updated design (Material Design) is mostly back-ported all the way to 2.3. The user experience is practically the same on 5.0 as pre-5.0.