I would expect that Leeds and Huddersfield pick up a sizeable proportion of fans from there too, so a limited impact at best. Admittedly, being from Ryhill originally, I'm one of those who proves your point!
I live in Wakefield south wakefield border line leeds. My dad was from Hemsworth origanally but always followed barnsley and took me to my 1st game in the promotion season under Wilson at 10 years old. We then got season tickets after that. But I know quite a few from wakefield who follow barnsley admittedly mostly leeds and a few Huddersfield but with the right marketing and results barnsley could draw even more support from the area I think.
Understandably the Cryne family will be taking some of that but the club certainly aren't going to be spending much on players and wages so now is the time I reckon. The West Stand is just old, it doesn't have any architectural value.