There are a fair number of new, clean-sheet designs which incorporated winglets from day 1, so it's not true to say that they are just a fix to improve things without a full recert.
Simply, they are one of the tools in the aircraft designers' toolbox. Sometimes they trade-off and buy their way on, sometimes they don't. Part of that trade-off can be tangibles (like weight and drag considerations), sometimes its intangibles ("all our aircraft have winglets, we aren't changing that" or "customers think winglets look nice" or whatever).
There's no technical justification for the winged lady on the bonnet of a Rolls Royce, but they still put one there.
Boeing CHOSE not to include winglets, based on a combination of engineering trade-off of the pros and cons, and that will have included things such as the corporate "look" of their aircraft (especially compared to AB), their comfort level with the technology, and so on.
To address the OP, supercritical (or pseudo-supercritical) wings and high aspect ratio wings have both been designed from the start with winglets, so those aren't reasons not to include them.