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It wouldn't have happened because the 6809 wasn't binary compatible with the 6800.

The 6809 was SOURCE compatible with the 6800 - you can assemble 6800 code on a 6809 assembler and it will run with perhaps very minor tweaks.

So?

Because none of the existing software would work. The idea of running a Rosetta-like feature on an 8-bit CPU isn't feasible. The Apple II eventually received an upgraded processor, the 65816, which was compatible with the 6502.

In those days nobody cared about binary compatibility. If you had an assembler and the source code you were all set.

Though a problem, as you point out, it still happened. The 6800 based SWTPC was followed by 6809 machines what need to have all their software reassembled.

On the other side of the cpu wars, all those 8080 machines moving on the Z80s got to keep all their binary software, which happened again for IBM PCs and clones as those evolved.

https://en.wikipedia.org/wiki/SWTPC_6800

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