Well.Fair points. You are correct, appealing to everyone with this system would be impossible, no one would ever be happy with the map/mod selection but I don't think that should stop us from trying when there are so many benefits to having something like this in place.
The problem with a career mode is the same as with pp: You can't find a one-size-fit-all solution.
Also, while it is true that many players play for pp, there are also many players with a less competitive mindset that are simply not as eye-catching as they won't really make an appearance in terms of "how can I improve the fastest in this game?"
Personally I'm a player that has a very off-meta skillset although pp and competition was always a partially driving factor for me.
Last but not least, if you look for these things you will already find things that have a similar design, be it Pokebis' Beginner Training (which is still a good starting point in spite of the old fashioned maps in it), RaneFire's lists, Akali's Jump Training practice (have the bookmark on my other PC), my own guide on building up a diverse skillset or this crazy guide by Korilak on stepping beyond the average. Not to mention the other stickies in Gameplay & Rankings and generally massive piles of thoughts on playstyles and philosophy although scattered.
There are dozens of reasons to play osu! and accordingly the motivation of players differs - I don't even want to start on different tastes of music which is a massive factor regarding what you will be motivated to play. It is for me at least: The fact that I'm really into low bpm and upbeat songs didn't occur to me that much before getting into osu! but whenever I'm listening to song somewhere and I think "wow, this is so cool, I could try to map this" it ends up in that nice range of effectively 120-160bpm. Guess what I'm best at compared to other players of my rank.
As such I think a career mode might be difficult to implement to everyone's happiness and pp at least leaves you the freedom of song choice.
I appreciate that there are resources available but you're assuming that new players will even look for them and if they do, that they'd even known where to look. I, like many others, at one point went searching for guides on how to improve at osu! and I haven't seen any of those links before (Though I'll be bookmarking them and forwarding them to new players in the future). Downloading and running the osu! client as a player completely new to the game you treat it as any other game, the community-driven aspects to this game are somewhat unique and intuitive, especially with a game that seems inherently single-player. By the time players would seek out these resources, many of them have already quit and for some of them it's just too little, too late. Bad habits form quickly and we should do something about that before, not treat the problem after.
While such resources as the ones you linked are useful, they still don't address all of the points I put across. The new-player experience in this game is awful and volatile. The reddit version of this post had a lot of people who have quit the game saying this likely would've kept them going to a point where they're proficient enough to make their own way from that point onwards. There is no standardisation of the new-player experience and there needs to be if osu! is to keep growing. Player retention in rhythm games is always shaky and throwing new players into the deep end with very little direction isn't helping.
There are a lot of motivations to play osu!, it's true. But for those looking for quantifiable, observable personal progress as many do with such a skill-oriented game the easy target is PP and you can see the result of that everywhere.
This isn't a perfect solution, but these are problems that need to be addressed somehow. Once they are we could see the active users skyrocket, especially if something like this is implemented with the release of osu!Lazer. The new client is bound to attract a lot of new players, but most of them won't stay for long unless we do something about it.