All of this costs money, in terms of either having more developers/testers or having longer development time. So, in order to justify this investment, the second platform must be way cheaper in order to cover costs for extra developers/development time. And if there is a such huge difference and second platform works great, then why still have support for first platform anyway. Ditch it, and you will save yourself some money.
You could be an ISV, but again, your software will be more expensive if you need to support two different platforms. Which means that your customers must be willing to pay for it. Which brings us to same conclusion, unless there is a big saving by running software on alternative platform, nobody will care.
Google's data centers collectively use more electricity than the state of Rhode Island, or about the same as the entire country of Costa Rica. Their electricity consumption has doubled in the last four years. At average wholesale prices, their annual electricity bill would be about a billion dollars. ARM isn't dramatically more efficient than x86 in most applications, but specialised ASICs can be orders of magnitude more efficient.
I'm not saying that nobody cares about the choice of architecture, I'm saying that major tech companies with vast quantities of servers are beginning to develop their own silicon with custom architectures and custom instruction sets, precisely because that's vastly more efficient than using a general-purpose architecture that happens to be popular in the wider software ecosystem. The fact that nobody else uses that special-purpose architecture is unimportant, because it is economically viable for them to invest in tooling and training to write and port software for these weird chips.