The evolution has been driven primarily by the available technology; only now are the operational aspects taking the lead.
Electronic displays for commercial aviation evolved from research. In the UK this included the Advance Fight Deck (BAC Weybridge) using 7 black and white CRTs.
Later, two ‘large size’ colour CRTs were flown in the RAE BAC 1-11.
IIRC NASA had similar developments.
The first commercial aircraft used small colour CRTs, 6x6in and 6x8in, slightly larger sizes may have been used in the 757 and A310. One of the first corporate aircraft was the HS 125.
Because of the small display area, only a limited amount of information could be displayed. The acceptability of various formats dominated many regulatory discussions, particularly airspeed (tape vs dial). With the advent of larger CRT displays and subsequently LED/LCD technology, then all instruments could be replaced.
Another technical driver was the system interface. Initially for simplicity (cost, risk), the existing analogue interfaces with sensors, controls, and recorders were retained.
Later, with newer aircraft designs or significant type update, fully digital interfaces and digital sensors / systems were used; this coincided with widespread use of digital auto flight.
I suspect that recent and evolving generations are influenced by the processing power and availability of input systems, FMS, etc, and not display size per se.
Some interesting developments include format options ‘back to dials’ (737); perhaps these are due to weaknesses in the displays’ HF interface (man-machine), or for operational commonality with existing ‘older’ aircraft.
There are also evolutions with display control interface, which again are driven by technology (cursor control; touch screen ?).
The new wide screen displays are generally used to replace the complete instrument panel; individual instruments are still outlined and operate in a similar manner to earlier designs.
There are some GA/corporate aircraft which attempt to use full screen displays of ‘the real world’ with conventional instrument overlays. My limited experience of these suggests a cautious implementation, avoiding too many changes at once, and remembering that new training/cross conversion is expensive, and that any time of significant change there is opportunity for error and a reduction in safety.
Categorisation by generation would be driven by technology and initially display size; whereas modern evolution is more likely to driven by operational and human considerations. The latter would include aircraft / operating environment (e.g. ATC) and the wider aspects of the total aviation operating system (operational situation, aircraft and system operation).
The retrofit marked should not be overlooked; here display size and system interface are major issues, as is cost/reliability which drives the need for change.