A few thoughts from a pianist with three professional music teachers in the family:
0) Someone already came up with a classical piano repertoire that perfectly increases in difficulty from "Primary" to Grade 10, in neat, progressive steps. If you look for sheet music for that Adele song, I guarantee you you'll never find a version that's appropriate for each of those levels. You might find several versions, each somewhere in the middle of that range, but no one will spend the time to interpret "Someone Like You" in eleven different ways--from an 8-bar melody up to some 15-page virtuosic piece--and then repeat that for every other song in the top 100. Can't even crowd-source that effort due to copyright.
1) You're vastly overestimating what a complete beginner can do. Remember, these are normally 5 year old kids with barely enough motor control to hold a pencil properly, no concept of beats or rhythm, etc. Pop music is usually easier, but it still takes years to work up to that Adele song. "What about adult students?" Adult students are even worse for quitting things early because it'll still take months to play anything remotely interesting and there's no one forcing them to continue. Proof: note how many adults will join and then quit the gym between New Years and February 1st because they're not seeing results quick enough.
2) To build on the above, piano has the shortest possible path to making in-tune sounds. Think how long it takes to learn to play even a single note on an oboe. You need the many lectures on posture and finger position--not to mention tons of boring practice!--just to stop squeaking.
3) How do you go about teaching Adele without explaining note names, how counting works (fractions!), and all the associated music theory? "I play, you copy?" That's not considered good music education. Kids get decent at rote memorization of finger patterns, and then plateuau at that (very, very low) level.
4) Beginners already start with "fun" music like Old McDonald, London Bridge, etc. Most of it is lame, but then again, kids might find recently-mainstream music like N'Sync lame too. Fashion moves pretty fast. On that note, copyright/royalties on modern music are a pretty good barrier to just playing whatever you want. (And improvising or playing "by ear" is a complete non-starter. That's the kind of thing you attempt after years of experience, not as a beginner.)
5) Trust me, teachers are intimately aware of the fine balance between motivating their kids with fun repertoire and actually teaching them something. Vocal teachers probably work that fine line the hardest since every pop song is fair game.
6) Pop songs in addition to classical repertoire is fine; not a single music teacher will object to this. My mother held three recitals a year (popular, christmas, classical) and kids held free reign over repertoire selection in two of the three (and the third was mostly a "trial run" of the pieces they'd have to play anyway for their upcoming piano exams). Pop songs as a replacement for classical repertoire is not fine. Adele wasn't thinking about teaching you anything about fingering or expression marks when she wrote her songs. On the other hand, Chopin/Liszt/Czerny/Debussy/etc. were writing with exactly those concerns in mind when they wrote their Etudes (read "studies"). They're written to be lyrical and interesting, but also to focus your practice towards a specific, technical challenge (perhaps one etude is just about fast arpeggios for the right hand, another etude for long trills with the weak fingers of the left hand, etc.)
7) Along the same lines, "classical" education isn't just Classical-era music, like Mozart. Good teachers want to expose you to lots of styles. Yes, every pianist has to play Bach--even if you hate Bach--because it's instrumental to learning about polyphony. But somewhere in that repertoire book there'll also be jazzy music, latin music, cinematic-style music, etc. written by still-living composers (although probably not as modern as Adele). In RCM books you'll find that kind of thing in sections 'D' and 'E'.
8) And yes, nobody likes scales, but drills are drills; you just have to accept that some parts are laborious and annoying. Athletes spend time running between plastic pylons and sweating it out at the gym because you can't efficiently build fundamentals just by playing games. You need a good mix of performance and practice. If you're only looking to "have fun", you can just stick to pick-up games with your friends (or teach yourself how to strum a few guitar chords), but if you're taking professional lessons in something there's an implied expectation that you're there to improve.
Obviously music is there to be enjoyed. And I'm certainly not claiming that traditional music education can't be improved. I just wanted to make the point that teachers already try to balance motivation with other goals, and that no activity worth doing is "all fun, all the time".