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I think in terms of efficiency and technology, the aircraft of today will have less radical improvement than, say, twenty-five years earlier. Aviation technology has progressed so much in the last one hundred years that there is only modest room for much improvement. Therefore, if Airbus and Boeing were to produce a small jet at different times, the terms of competition would be quite fair and equal. The better design, of course would prevail.
However, whoever comes up with a new design first would more likely gain a bigger market share as airlines would be reluctant to switch to the later-introduced model, preferring to stick with the earlier one (if they purchased it).