I think Rooney was an excellent player for us for around 8 years (2004-2012.) If you wish to argue that he wasn't consistently excellent at the beginning, then you could also argue that he was sporadically excellent during Moyes tenure (so give or take, 8 years of top-level service.) I don't think it's pushing it to suggest that he could have been better, but nevertheless he's been a top contributor to this club. Arguments as to whether he was 'world class' or better than Shevchenko are just that - arguments, and they grow tiresome very quickly. What can't be argued is his goal haul, which is very impressive (and the bulk of which came during that aforementioned quality spell - he hasn't exactly been filling the net in recent year, thereby inflating his number.) So for me, I'd like to see him break the record, as it would be a reflection on his contributions during a golden era for the club (2006-2013,) a period in which he was fairly often played out of his best position (or perhaps it's better to say that he didn't really have a defined position,) and unlike now, made little fuss about it.
I think he's been an absolute prat at times (for me, the worst was his non-celebrating of goals in the opening of the 2013-4 season,) and he's certainly past his best-by date now. However, it's revisionist in the extreme to suggest he hasn't been a great player for the club. Does this make him a legend? I'm not sure, as the criteria for that word often seems to go beyond matter on the pitch. But I do know that there was a time when the thought of losing him was pretty scary, and that in itself speaks volumes.