Sequels can be good or bad but I think specifically for the Digimon franchise it works better as stand alone seasons work best from a writing stand point. Part of the reason I feel 02 didn't work was because you had a cast of new characters, evolutions, & rules as to how the universe works piled on top of the of Adventure universe. As a result you have the writers trying to rewrite aspects of the Adventure universe to accommodate the 02 universe. Now compare that to two random, unrelated seasons like Tamers & Frontier. Both had their own set of rules & how things work & could take those concepts as far as they'd like without having to worry about what the season before did.