There are a lot of ways stats increasing from activity could go way wrong, for your idea of CHA people would probably go to a secluded area and spam that /smile command you mentioned, it could be worked around, if you could only get 1-2 cha points per person, ever. For INT and the books, people could write a book of gibberish, or scroll up and down the book, you dont even have to pay attention to get that one up, with your theory. For the other three, auto-run, others might make a bot to jump up and down, or run in a circle, or some nonsense..Anyways, its to exploitable for it to be implemented without being perfected in every way..which I have no idea how the devs could ever do...
A [IMO] better way to get stats up than activity, is still activity But you still have to train, if the various skills had a parental stat, aka Axe - Strength Dagger- Agility. HA - ENdurance, Magic skills already have a parental atribute which would be applied to those magic skills.
Anyways the point of that, is every time you train a skill in combat or magic, or job or whatever, you would get an associated stat point(s.) This way its not exploitable, but its also not limited, You wouldnt have to hassle for winch quests, to get your stats over 150-200 You could train your way to 400, in any hobby you might have. Things like crafting would do strength and intelligence, cooking might be intelligence and agility, via slicing things, you get fast at it with practice. So the stats could be raised from all around, The other plus from this is that new players wouldnt have to save money for both stats and skills the could focus on just skills, it saves time, and gives them more time to do what they want, be it train or rp,