Bullethead does make an excellent point; most pilots do not find it very difficult to transition from new technology to old technology. It is, however, very difficult to reach pilot consensus about various design options, as most the design engineers will readily attest. There are differences of opinion between young and old pilots, civil and military pilots, and line pilot versus pilots in management. There are differences of opinion between design authority standards (e.g. French and US) and government regulations. There are often several very good solutions to the same design question. Engineers for the most part would prefer to search for the most elegant solution, but are often required to do so within externally imposed cost, schedule, and regulatory constraints.
O-I ADI rather than an I-O ADI? The market is always the final arbitrator. One design, often regardless of merit, will be selected over the other by the market. The market might be forced to make a selection because a government imposes a design, or because of cost, or because of senior management fiat; but the design or product is rarely selected because it is the most elegant solution.