Cas are "severely limited" by what?
As a "classy" brand the club don't "punch above their weight" quite the reverse,
They attract a weight of support that in relation to the size of the town indicates one in six Cas people support the club.
Which isn't the case at all. People travel from far and wide (relatively speaking) to see an exciting club/team.
AFAIK several people In Selby are Leeds fans several are Cas fans.
If we work on your assumptions then Everton should not exist?
You say "The likes of Salford have huge amounts of untapped potential".
They have had this potential for years, but they are not "tapping it".
The question for you is this.
If Salford and Castleford were grand finalists and Challenge cup finalists ever year for the next three years which club would have the highest attendances????
Because as it stands both clubs have no chance of any final appearance yet the small town produces bigger attendances than the big dual city...
Demographics, geography, local competition, commercial viability, profile, finance, facilities, investment, sponsorship .... I could go on.
Unless Castleford can find a local born multi millionaire who's prepared to invest on the Ken Davy scale and show the same level of patience then they're unlikely to ever reach let alone sustain being a top club. Of course they might manage to string some performances together that gets them to a semi final or the 2nd week of the play offs but it will be a flash in the pan rather than a sustained, year in year out challenge for honours. They may manage to homegrow some amazing young players who mature together and give them a strong team but, for all the reasons above, they don't have the wherewithal to keep them.
Every club attracts fans from a wider area but unless you're Man Utd it's likely to account for a small percentage of your overall attendance. The fact that they attract 1 in 6 of their population is both great and not great, it's a credit that they have such pull in their home Town but at the same time it suggests they've squeezed almost all they can realistically expect from Castleford. Without somehow becoming the Classy Cas of old or transforming themselves into annual contenders they're unlikely to overcome their limitations and draw the huge and significant numbers of supporters from outside the Town, it's a vicious circle that's become more vicious in modern top flight sport.
Let's not use Premier League clubs as a comparison because the forces and finances in play render it pointless they aren't even in the same Universe!
In my honest opinion if Salford were to do a Leeds and mount a ten year attack on Challenge Cup and Grand Finals then they'd attract more fans than Castleford and what's more they'd become immensely more attractive to new big money investors which would allow them to sustain their prominence.