In those cases, the switched-to architecture could emulate the previous one with acceptable performance. For the hypothetical amd64 -> aarch64 transition, I'm unaware of the existence of software, silicon or the combination of the two which could emulate the former on the latter while staying in a reasonable power envelope.

Emulation is non-optional if one wants to avoid splitting the ecosystem.

Both times were out of necessity though.

Arguably, the necessity here would be energy consumption, thermal performance and potentially speed?

That's not really necessity unless you can show that the current values for those 3 are insufficient and that an Axx transition will produce big gains in at least 2 of those 3 areas, is it? Even if native apps compiled and written for the Axx architecture have better energy and thermal performance, what about all the x86 apps that have to be emulated? There'll be a tax for that. It could be worse. Intel-based macs are probably wasting some power that Axx would theoretically not waste, and Axx might theoretically have better thermals, but there's not some massive demand for Apple to upend the whole ecosystem like that. Does Apple have competition threatening to steal their lunch because Macbooks' battery life isn't long enough? Not really, no?

I did say ‘arguably‘ ;)

> Even if native apps compiled and written for the Axx architecture have better energy and thermal performance, what about all the x86 apps that have to be emulated? There'll be a tax for that.

Initially, probably. Think of the long game. This is something they have already done, twice. They have the experience and expertise to make this less impactful.

> Even if native apps compiled and written for the Axx architecture have better energy and thermal performance, what about all the x86 apps that have to be emulated? There'll be a tax for that.

Longer battery life is always preferable in portable devices, no? Alternatively, what about the reduction in the need for the materials, therefore the environmental impact, used in making batteries for laptops? Apple want to be the greenest manufacturer.