The Power Base Convertor is technically compatible with the Model 2. It just does not physically fit the console without either modifying or removing the adapter plastic housing.
The problem with backwards compatibility is that it forces the new console to retain similar hardware (or software emulation, as the PS3 did to support PS1 games as well as PS2 emulation for a time after the hardware compatibility was removed but before support was entirely removed).
That can sometimes be detrimental to the new console, as reportedly the 7800 was left using the same sound chip as the 2600, which would have been nearly a decade old and thus significantly behind the competition by the time the console launched.
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