Most big malls like Central Festival are not made up of 'small shop owners', although they do have some, plus the kiosks. Big malls are mostly made up of chains--both for stores, restaurants, movie theaters, banks, pharmacies, and services. Whether I am at Central Festival Pattaya or Rama 9 Mall in Bangkok, there's Uniqlo, H&M, Power Buy, all the same banks, True, AIS, Fuji, Pizza Company, Boots, Daiso, Zen, Sketchers, SF Cinema, etc., etc. If it's a chain at one, it's likely to be at the other.
Terminal 21 is a bit different from most traditional malls as it has made the effort to set itself apart with its travel theme concept and it does have a larger number of independent shops on the side aisles. The main aisles still carry mostly chains. You're correct that it's the little shop owners that are more likely to be changing hands, rather than the better-financed chains with their established customer bases. A 'little watch shop' going out of business will have little impact on a mall--and, at some point, another little shop will take its place. That's business.