RISC-V. But really ARM is a totally legit choice at the moment, not out of touch at all, and I say that working for a RISC-V company.

The entire ARM ecosystem is far more mature than RISC-V, plus the main RPis are ARM so they have experience with it and probably access to ARM people.

The main reasons to use RISC-V currently are cost and customisability. There are also some quite high quality open source RISC-V cores you can use off the shelf.

RISC-V is only really directly helpful for those building their own silicon (sure "anyone" can implement a RISV-V core in a FPGA but that won't give you the performance of an ASIC).

If you just want to buy a chip and put it on your own board you don't really care that much about the ISA as long as the toolchain, bootloader, kernel support is there and stable (which is pretty much the case with RISC-V now).

But if you're desiging your own silicon the ability to make or modify an existing open source RISC-V implementation can give you the flexibility that currenlty only the very largest players in the ARM ecosystem that have architecture licenses have (I think that's pretty much limited to Apple, Qualcomm, Samsung).

I expect the smaller but important players like NXP, ST, TI will be bringing out RISC-V chips taking advantage of this.

ARM have also been talking about making their fees dependant on the price of the final product so that could also push people to RISC-V, not just to pay less but also to not have to jump through the hoops that would be required to provide all the information to do that.