Rust will not reduce platforms

This applies to virtually all drivers for hardware that isn't in the SoC itself (eg chips connected to the CPU using busses like I2C / SPI / PCI / USB ).

Even when the hardware is actually inside the SoC it's quite common for IP blocks to be reused in multiple SoCs, even ones from different manufacturers (because manufacturers often buy the IP for an ethernet controller, USB controller or whatever and integrate it in their SoC). In that case the register interface is the same so the driver code is the same but the registers will be at different addresses (and that's taken care of by injecting the appropriate base address by DT / ACPI)

So, in many cases, having drivers in Rust will impact Linux support for platforms that don't yet have a Rust implementation. And while it is indeed possible to have competing implementations this usually frowned upon in the kernel for duplication / maintenance reasons and usually exists only temporarilly.