ARM64, at least, is moving towards UEFI/ACPI for booting/peripheral support, so ideally a lot of things will "just work" once people are shipping mobos/silicon with it enabled (and they already are).

That said it's unclear what the volume on those parts will be once higher-grade CPUs come out, which will impact pricing/availability. And newer parts are going to be necessary for competitive performance/power. If the only place you can get a 14nm Thunder X2 is some high-end SuperMicro workstation, it'll be somewhat limiting in the end, anyway. (You can buy a Thunder X1 prebuilt desktop on the order of $1500 USD right now, at least.)

ARMv7 of course is a different bag of worms; Mali tends to ruin everything. But if you can avoid it, a lot of systems can get by with upstream U-Boot and upstream Linux... You can even get some cheap Asus chromebook laptops that can run libreboot and upstream everything (including WiFi if you buy a dongle.)

That’s exactly why the Arm servers adopt the same specs or functional equivs to x86. A single binary distribution of an OS will boot on any SBSA compatible machine - because all the weird bits are hidden through ACPI, UEFI and PSCI. No BSPs.

I’d go as far as saying Arm servers are compatible to x86 designs at a platform level (eg. how PCIe and config space are wired up, how caches and DMA work, NUMA, SMP, MSI(X), and so on....

Is there any hope that it could be used to some extent on smartphones? Or maybe there is a similar effort in that space?

I understand that some works may be hard to miniaturize.

IMO yes. Although SBSA and SBBR target the servers, already smaller gear (think vCPE, set top box and IoT gateway chips) are adopting the specs as well, and I anticipate adoption everywhere except devices where a) cost is everything b) power and perf need to be squeezed out even at the cost of maintainability. At some point following the everything-needs-its-own-BSP mentality for embedded SoCs will start hurting the bottom line, because there will be plenty of competitors willing to make more “normal”-looking gear.