That's a huge time spread you give there casual observer and it well illustrates the problems. The depreciation period for aeroplanes is a good long twenty five years - there's no point in quibbling about the relevance of selections made in the eighties to today's operations.
To illustrate further, we selected the A319 in 2000 because it was right for the market and route plans made at the time. After setting the specifications, the order was postponed for a number of reasons. In the meantime the base-line specifications changed and market research, moving with the times (SARS, oil prices, economic externalities etc. ) suggested changes to the originally planned route structure. The 319s were delivered in 2003, to the original specs, and A320s were ordered with ETOPS capability and different tech. specs. (the original equipment specs were no longer available). Meanwhile we still have Boeing 767s on the long-haul routes. Not the best choice for the routes we fly perhaps, but they do the job after a fashion and they're simply carried over from earlier days.
You buy aeroplanes in a dynamic business environment and have to live with your choices. Just like any other long-term investment.