Akira Toriyama actually does vary his style (while keeping his style). For example: Look at his dragon quest cover art.
Hahaha , yes , I'm well aware of that . I was just poking out a joke out of this current discussion (I guess I'll stick to my 'natural' jokes instead ...)
Okay , on to my personal opinion .
I always had this saying in my mind ,'If it's not broken yet , why fix it ?' . In my opinion , he's probably afraid of 'breaking it' & the reaction he'll get if he does 'break it' . Everytime one risk implementing something that is out of ones 'comfort zone' , both to oneself & to the fans , one needs to expect the unexpected ... not many like such risk though ...
So , yeah ... big nose ... okay . So he draws lots of ... 'ugly' characters ... okay . Outdated art style ... okay .
His 'ugly' characters ain't ugly at all & I can easily point out a good amount of newer generations manga artists who're guilty for the opposite of this guy , for drawing mostly 'pretty' characters . Heck , I can also say Range Murata (not really a manga artist . More like a designer for animes) for drawing mostly chlid-like anime characters (okay ...) or CLAMP for drawing mostly elongated characters (okay ...).
He chose to draw it in such a way because he chose to & although he draws a lot of it , that does not mean that he cannot draw different art styles as well . But he chose not to as he feels that it does not represent 'him' in the world of manga/comic , but there will be times that he might've feel like the time is right to add something different in his works to give it a bit of 'variety' , 'change' etc. , but not so much that it smoulders his signature art style .
Sure , like my personal favorite manga artist , Kosuke Fujishima , who opt to fix his art style at that time , Naoki Urasawa's art style can be improved , to me , personally , in my opinion (there's no such thing as the end of the road when it comes to art) but this is up to him if he wants to or not - no one but him/herself should dictate his/her art style simply because it's not appealing anymore , only the technical flaws that lies underneath his art style , which is not 'broken' at all (he is , after all , a veteran manga artist) . Art style is subjective , there is no 'right' or 'wrong' way to it .
Sure , it does not appeal to some of you , so do I (but I prefer to like it at the same time , that's just being me) . So let's just leave it at that . He probably never had that intention to create something that will appeal to readers like us (you can't appease all , it's completely impossible) , so let's respect that decision of his . It's his work after all , he has the control over it , not us .
... I'd sure launch my pencil missiles at them if they ask me to change MY art style for 'selfish' reasons ... I need to 'purge' myself , too high on Caffeine - I feel like ranting =_= .