^Well, now debating upon this thing would lead us to a religious debate which is forbidden on this forum. So, I would explain it in a simpler way to clear your point:
Actually, our banking system is based upon international banking systems. This kind of system was introduced by Jews. According to this system, the money deposited in the bank by opening saving accounts (not current accounts) is utilized by the banking authorities to provide loans to the debtors (people or company), which govern their business or etc from these and return the loan money back to the bank along with the interest (suud) according to the interest rates (suud ki sharah). Now, the profit earned in the form of interest (suud) is then shared with the account holders according to their interest rates (munafa dene ki sharah). Now, according to the islamic point of view, interest (suud) is entirely haram, so taking profit from such accounts is also haram (because banking system that I have explained above is suudi and haram).
Now, coming to the Islamic banking, here the bank invests the money deposited in their bank by account holders in some kind of business. In such kind of business, profit or loss earned is shared with the account holders according to their profit ratios. Now, this kind of banking is Halal and according to the Islamic teachings. But one more thing, if bank is governing a business which is illegal according to our religion, then again, taking profit from them is Haram.
And since you said molvis of 1950s-1980s were not good Muslims, yes, they were not. Those were actually ''chawal khaney walay mullas.'' (An ordinary man has not brought such disaster to Islam what this type of mullas have).