More than two dozen bank investors and analysts gathered for a luncheon with Valley National Bancorp Chief Executive Ira Robbins last month, a few days after a fellow regional lender stunned Wall Street by posting an unexpected loss and slashing its dividend. Most of them wanted to know how New York Community Bancorp the nation’s 28th-biggest bank, had found itself in this position less than a year after several other regionals failed. The banking industry’s middle class is shrinking.