On current form, I seriously suggest you are writing off Billy far too quickly.
I would point out to you that the Leeds side in the championship 3 years ago being a shadow of the side it is today, and who saw fit to deploy young William as a left sided midfielder for the most part !!!
I take your points as regards the others and that if promoted, investment in the squad will need to be significant.
But, IMHO, we do currently have 4 or 5 players of genuine Championship quality. Maybe a few others can develop too (Whiteman / Lavery).
UTB
Obviously Billy's enthusiasm & goals have fuelled our form and I appreciate that if we go up , his contribution will have been crucial.
However , it reminds me of Stan in 1990 , aged 32 , leading us into the Prem after 7 years of stalwart service , but knowing that his heroic status can't defeat the aging process in a more demanding league.
I don't think that Billy would look out of place , but I can't see him carrying the scoring responsibility.
It's even harder for a striker who depends on reflexes , not height or pace , to make the step up at 31.
No true Blade can dislike Billy , but this isn't an emotional assessment , just business.
Who could play - effectively - in the starting 11 - if we got to the Championship tomorrow ?
Moore , JOC , EEL if we get him , Freeman if on current form , Lafferty ,
Coutts , Fleck , Duffy (for short-term , Samir is his replacement) ,
That's it.
Who could be useful bench/squad names ?
Basham (think Monty In the Prem) ,
Lavery (if he develops as I think he will) ,
Billy (because we can't afford to buy 3 new strikers) ,
A few promising youngsters.
If CW thinks this season is a challenge , his horizon could be about to explode in the summer.
I hope he gets a lovely long holiday somewhere restful in May ☺.